lib/adapters/http/index.js

"use strict";

var CHANGES_BATCH_SIZE = 25;
var MAX_SIMULTANEOUS_REVS = 50;

var supportsBulkGetMap = {};

// according to http://stackoverflow.com/a/417184/680742,
// the de facto URL length limit is 2000 characters.
// but since most of our measurements don't take the full
// URL into account, we fudge it a bit.
// TODO: we could measure the full URL to enforce exactly 2000 chars
var MAX_URL_LENGTH = 1800;

var uuid = require('../../deps/uuid');
var pick = require('../../deps/pick');
var filterChange = require('../../deps/filterChange');
var coreAdapterFun = require('../../deps/adapterFun');
var explainError = require('../../deps/ajax/explainError');
var binStringToBluffer =
  require('../../deps/binary/binaryStringToBlobOrBuffer');
var b64StringToBluffer =
  require('../../deps/binary/base64StringToBlobOrBuffer');
var utils = require('../../utils');
var Promise = require('../../deps/promise');
var clone = require('../../deps/clone');
var parseUri = require('../../deps/parseUri');
var getArguments = require('argsarray');
var base64 = require('../../deps/binary/base64');
var btoa = base64.btoa;
var atob = base64.atob;
var errors = require('../../deps/errors');
var log = require('debug')('pouchdb:http');
var createMultipart = require('../../deps/ajax/multipart');
var blufferToBase64 = require('../../deps/binary/blobOrBufferToBase64');
var parseDoc = require('../../deps/docs/parseDoc');
var bulkGetShim = require('../../deps/bulkGetShim');
var flatten = require('../../deps/flatten');

function readAttachmentsAsBlobOrBuffer(row) {
  var atts = row.doc && row.doc._attachments;
  if (!atts) {
    return;
  }
  Object.keys(atts).forEach(function (filename) {
    var att = atts[filename];
    att.data = b64StringToBluffer(att.data, att.content_type);
  });
}

function encodeDocId(id) {
  if (/^_design/.test(id)) {
    return '_design/' + encodeURIComponent(id.slice(8));
  }
  if (/^_local/.test(id)) {
    return '_local/' + encodeURIComponent(id.slice(7));
  }
  return encodeURIComponent(id);
}

function preprocessAttachments(doc) {
  if (!doc._attachments || !Object.keys(doc._attachments)) {
    return Promise.resolve();
  }

  return Promise.all(Object.keys(doc._attachments).map(function (key) {
    var attachment = doc._attachments[key];
    if (attachment.data && typeof attachment.data !== 'string') {
      return blufferToBase64(attachment.data).then(function (b64) {
        attachment.data = b64;
      });
    }
  }));
}

// Get all the information you possibly can about the URI given by name and
// return it as a suitable object.
function getHost(name) {
  // Prase the URI into all its little bits
  var uri = parseUri(name);

  // Store the user and password as a separate auth object
  if (uri.user || uri.password) {
    uri.auth = {username: uri.user, password: uri.password};
  }

  // Split the path part of the URI into parts using '/' as the delimiter
  // after removing any leading '/' and any trailing '/'
  var parts = uri.path.replace(/(^\/|\/$)/g, '').split('/');

  // Store the first part as the database name and remove it from the parts
  // array
  uri.db = parts.pop();

  // Restore the path by joining all the remaining parts (all the parts
  // except for the database name) with '/'s
  uri.path = parts.join('/');

  return uri;
}

// Generate a URL with the host data given by opts and the given path
function genDBUrl(opts, path) {
  return genUrl(opts, opts.db + '/' + path);
}

// Generate a URL with the host data given by opts and the given path
function genUrl(opts, path) {
  // If the host already has a path, then we need to have a path delimiter
  // Otherwise, the path delimiter is the empty string
  var pathDel = !opts.path ? '' : '/';

  // If the host already has a path, then we need to have a path delimiter
  // Otherwise, the path delimiter is the empty string
  return opts.protocol + '://' + opts.host + ':' + opts.port + '/' +
         opts.path + pathDel + path;
}

// Implements the PouchDB API for dealing with CouchDB instances over HTTP
function HttpPouch(opts, callback) {
  // The functions that will be publicly available for HttpPouch
  var api = this;

  // Parse the URI given by opts.name into an easy-to-use object
  var getHostFun = getHost;

  // TODO: this seems to only be used by yarong for the Thali project.
  // Verify whether or not it's still needed.
  /* istanbul ignore if */
  if (opts.getHost) {
    getHostFun = opts.getHost;
  }

  var host = getHostFun(opts.name, opts);
  var dbUrl = genDBUrl(host, '');

  opts = clone(opts);
  var ajaxOpts = opts.ajax || {};

  api.getUrl = function () { return dbUrl; };
  api.getHeaders = function () { return ajaxOpts.headers || {}; };

  if (opts.auth || host.auth) {
    var nAuth = opts.auth || host.auth;
    var token = btoa(nAuth.username + ':' + nAuth.password);
    ajaxOpts.headers = ajaxOpts.headers || {};
    ajaxOpts.headers.Authorization = 'Basic ' + token;
  }

  function ajax(userOpts, options, callback) {
    var reqAjax = userOpts.ajax || {};
    var reqOpts = utils.extend(clone(ajaxOpts), reqAjax, options);
    log(reqOpts.method + ' ' + reqOpts.url);
    return utils.ajax(reqOpts, callback);
  }

  function ajaxPromise(userOpts, opts) {
    return new Promise(function (resolve, reject) {
      ajax(userOpts, opts, function (err, res) {
        if (err) {
          return reject(err);
        }
        resolve(res);
      });
    });
  }

  function adapterFun(name, fun) {
    return coreAdapterFun(name, getArguments(function (args) {
      setup().then(function (res) {
        return fun.apply(this, args);
      }).catch(function(e) {
        var callback = args.pop();
        callback(e);
      });
    }));
  }

  var setupPromise;

  function setup() {
    // TODO: Remove `skipSetup` in favor of `skip_setup` in a future release
    if (opts.skipSetup || opts.skip_setup) {
      return Promise.resolve();
    }

    // If there is a setup in process or previous successful setup
    // done then we will use that
    // If previous setups have been rejected we will try again
    if (setupPromise) {
      return setupPromise;
    }

    var checkExists = {method: 'GET', url: dbUrl};
    var create = {method: 'PUT', url: dbUrl};
    setupPromise = ajaxPromise({}, checkExists).catch(function(err) {
      if (err && err.status && err.status === 404) {
        // Doesnt exist, create it
        explainError(
          404, 'PouchDB is just detecting if the remote exists.'
        );
        return ajaxPromise({}, create);
      } else {
        return Promise.reject(err);
      }
    }).catch(function(err) {
      // If we get an authorisation error
      if (err && err.status && err.status === 401) {
        return ajaxPromise({}, checkExists);
      }
      // If we try to create a database that already exists
      if (err && err.status && err.status === 412) {
        return true;
      }
      return Promise.reject(err);
    });

    setupPromise.catch(function() {
      setupPromise = null;
    });

    return setupPromise;
  }

  setTimeout(function() {
    callback(null, api);
  });

  api.type = function () {
    return 'http';
  };

  api.id = adapterFun('id', function (options, callback) {
    ajax({}, {method: 'GET', url: genUrl(host, '')}, function (err, result) {
      var uuid = (result && result.uuid) ?
        (result.uuid + host.db) : genDBUrl(host, '');
      callback(null, uuid);
    });
  });

  api.request = adapterFun('request', function (options, callback) {
    options.url = genDBUrl(host, options.url);
    ajax({}, options, callback);
  });

  // Sends a POST request to the host calling the couchdb _compact function
  //    version: The version of CouchDB it is running
  api.compact = adapterFun('compact', function (opts, callback) {
    ajax(opts, {
      url: genDBUrl(host, '_compact'),
      method: 'POST'
    }, function () {
      function ping() {
        api.info(function (err, res) {
          if (!res.compact_running) {
            callback(null, {ok: true});
          } else {
            setTimeout(ping, opts.interval || 200);
          }
        });
      }
      // Ping the http if it's finished compaction
      ping();
    });
  });

  api.bulkGet = coreAdapterFun('bulkGet', function (opts, callback) {
    var self = this;

    function doBulkGet(cb) {
      var params = [];
      if (opts.revs) {
        params.push('revs=true');
      }
      if (opts.attachments) {
        params.push('attachments=true');
      }
      params = params.join('&');
      if (params !== '') {
        params = '?' + params;
      }

      ajax({}, {
        headers: host.headers,
        url: genDBUrl(host, '_bulk_get' + params),
        method: 'POST',
        body: { docs: opts.docs}
      }, cb);
    }

    function doBulkGetShim() {
      if (!opts.docs.length) {
        return callback(null, {results: []});
      }

      // avoid "url too long error" by splitting up into multiple requests
      var batchSize = MAX_SIMULTANEOUS_REVS;
      var numBatches = Math.ceil(opts.docs.length / batchSize);
      var numDone = 0;
      var results = new Array(numBatches);

      function onResult(batchNum) {
        return function (err, res) {
          // err is impossible because shim returns a list of errs in that case
          results[batchNum] = res.results;
          if (++numDone === numBatches) {
            callback(null, {results: flatten(results)});
          }
        };
      }

      for (var i = 0; i < numBatches; i++) {
        var subOpts = pick(opts, ['revs', 'attachments']);
        subOpts.docs = opts.docs.slice(i * batchSize,
          Math.min(opts.docs.length, (i + 1) * batchSize));
        bulkGetShim(self, subOpts, onResult(i));
      }
    }

    // mark the whole database as either supporting or not supporting _bulk_get
    var dbUrl = genUrl(host, '');
    var supportsBulkGet = supportsBulkGetMap[dbUrl];

    if (typeof supportsBulkGet !== 'boolean') {
      // check if this database supports _bulk_get
      doBulkGet(function (err, res) {
        /* istanbul ignore else */
        if (err) {
          /* istanbul ignore else */
          if (Math.floor(err.status / 100) === 4) { // 40x
            supportsBulkGetMap[dbUrl] = false;
            explainError(
              err.status,
              'PouchDB is just detecting if the remote ' +
              'supports the _bulk_get API.'
            );
            doBulkGetShim();
          } else {
            callback(err);
          }
        } else {
          supportsBulkGetMap[dbUrl] = true;
          callback(null, res);
        }
      });
    } else if (supportsBulkGet) {
      /* istanbul ignore next */
      doBulkGet(callback);
    } else {
      doBulkGetShim();
    }
  });

  // Calls GET on the host, which gets back a JSON string containing
  //    couchdb: A welcome string
  //    version: The version of CouchDB it is running
  api._info = function (callback) {
    setup().then(function() {
      ajax({}, {
        method: 'GET',
        url: genDBUrl(host, '')
      }, function (err, res) {
        /* istanbul ignore next */
        if (err) {
        return callback(err);
        }
        res.host = genDBUrl(host, '');
        callback(null, res);
      });
    }).catch(callback);
  };

  // Get the document with the given id from the database given by host.
  // The id could be solely the _id in the database, or it may be a
  // _design/ID or _local/ID path
  api.get = adapterFun('get', function (id, opts, callback) {

    // List of parameters to add to the GET request
    var params = [];

    // If it exists, add the opts.revs value to the list of parameters.
    // If revs=true then the resulting JSON will include a field
    // _revisions containing an array of the revision IDs.
    if (opts.revs) {
      params.push('revs=true');
    }

    // If it exists, add the opts.revs_info value to the list of parameters.
    // If revs_info=true then the resulting JSON will include the field
    // _revs_info containing an array of objects in which each object
    // representing an available revision.
    if (opts.revs_info) {
      params.push('revs_info=true');
    }

    // If it exists, add the opts.open_revs value to the list of parameters.
    // If open_revs=all then the resulting JSON will include all the leaf
    // revisions. If open_revs=["rev1", "rev2",...] then the resulting JSON
    // will contain an array of objects containing data of all revisions
    if (opts.open_revs) {
      if (opts.open_revs !== "all") {
        opts.open_revs = JSON.stringify(opts.open_revs);
      }
      params.push('open_revs=' + opts.open_revs);
    }

    // If it exists, add the opts.rev value to the list of parameters.
    // If rev is given a revision number then get the specified revision.
    if (opts.rev) {
      params.push('rev=' + opts.rev);
    }

    // If it exists, add the opts.conflicts value to the list of parameters.
    // If conflicts=true then the resulting JSON will include the field
    // _conflicts containing all the conflicting revisions.
    if (opts.conflicts) {
      params.push('conflicts=' + opts.conflicts);
    }

    // Format the list of parameters into a valid URI query string
    params = params.join('&');
    params = params === '' ? '' : '?' + params;

    id = encodeDocId(id);

    // Set the options for the ajax call
    var options = {
      method: 'GET',
      url: genDBUrl(host, id + params)
    };

    function fetchAttachments(doc) {
      var atts = doc._attachments;
      var filenames = atts && Object.keys(atts);
      if (!atts || !filenames.length) {
        return;
      }
      // we fetch these manually in separate XHRs, because
      // Sync Gateway would normally send it back as multipart/mixed,
      // which we cannot parse. Also, this is more efficient than
      // receiving attachments as base64-encoded strings.
      return Promise.all(filenames.map(function (filename) {
        var att = atts[filename];
        var path = encodeDocId(doc._id) + '/' + encodeAttachmentId(filename) +
          '?rev=' + doc._rev;
        return ajaxPromise(opts, {
          method: 'GET',
          url: genDBUrl(host, path),
          binary: true
        }).then(function (blob) {
          if (opts.binary) {
            return blob;
          }
          return blufferToBase64(blob);
        }).then(function (data) {
          delete att.stub;
          delete att.length;
          att.data = data;
        });
      }));
    }

    function fetchAllAttachments(docOrDocs) {
      if (Array.isArray(docOrDocs)) {
        return Promise.all(docOrDocs.map(function (doc) {
          if (doc.ok) {
            return fetchAttachments(doc.ok);
          }
        }));
      }
      return fetchAttachments(docOrDocs);
    }

    ajaxPromise(opts, options).then(function (res) {
      return Promise.resolve().then(function () {
        if (opts.attachments) {
          return fetchAllAttachments(res);
        }
      }).then(function () {
        callback(null, res);
      });
    }).catch(callback);
  });

  // Delete the document given by doc from the database given by host.
  api.remove = adapterFun('remove',
      function (docOrId, optsOrRev, opts, callback) {
    var doc;
    if (typeof optsOrRev === 'string') {
      // id, rev, opts, callback style
      doc = {
        _id: docOrId,
        _rev: optsOrRev
      };
    } else {
      // doc, opts, callback style
      doc = docOrId;
      callback = opts;
      opts = optsOrRev;
    }

    var rev = (doc._rev || opts.rev);

    // Delete the document
    ajax(opts, {
      method: 'DELETE',
      url: genDBUrl(host, encodeDocId(doc._id)) + '?rev=' + rev
    }, callback);
  });

  function encodeAttachmentId(attachmentId) {
    return attachmentId.split("/").map(encodeURIComponent).join("/");
  }

  // Get the attachment
  api.getAttachment =
    adapterFun('getAttachment', function (docId, attachmentId, opts,
                                                callback) {
    var params = opts.rev ? ('?rev=' + opts.rev) : '';
    var url = genDBUrl(host, encodeDocId(docId)) + '/' +
      encodeAttachmentId(attachmentId) + params;
    ajax(opts, {
      method: 'GET',
      url: url,
      binary: true
    }, callback);
  });

  // Remove the attachment given by the id and rev
  api.removeAttachment =
    adapterFun('removeAttachment', function (docId, attachmentId, rev,
                                             opts, callback) {

      var url = genDBUrl(host, encodeDocId(docId) + '/' +
      encodeAttachmentId(attachmentId)) + '?rev=' + rev;

    ajax(opts, {
      method: 'DELETE',
      url: url
    }, callback);
  });

  // Add the attachment given by blob and its contentType property
  // to the document with the given id, the revision given by rev, and
  // add it to the database given by host.
  api.putAttachment =
    adapterFun('putAttachment', function (docId, attachmentId, rev, blob,
                                                type, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = type;
      type = blob;
      blob = rev;
      rev = null;
    }
    var id = encodeDocId(docId) + '/' + encodeAttachmentId(attachmentId);
    var url = genDBUrl(host, id);
    if (rev) {
      url += '?rev=' + rev;
    }

    if (typeof blob === 'string') {
      var binary;
      try {
        binary = atob(blob);
      } catch (err) {
        // it's not base64-encoded, so throw error
        return callback(errors.error(errors.BAD_ARG,
                        'Attachments need to be base64 encoded'));
      }
      blob = binary ? binStringToBluffer(binary, type) : '';
    }

    opts = {
      headers: {'Content-Type': type},
      method: 'PUT',
      url: url,
      processData: false,
      body: blob,
      timeout: ajaxOpts.timeout || 60000
    };
    // Add the attachment
    ajax({}, opts, callback);
  });

  // Add the document given by doc (in JSON string format) to the database
  // given by host. This fails if the doc has no _id field.
  api.put = adapterFun('put', function(doc, opts, callback) {
    if (typeof doc !== 'object' || Array.isArray(doc)) {
      return callback(errors.error(errors.NOT_AN_OBJECT));
    }

    doc = clone(doc);

    preprocessAttachments(doc).then(function () {
      // check for any errors
      // TODO: rename this function
      parseDoc.invalidIdError(doc._id);

      // List of parameter to add to the PUT request
      var params = [];

      // If it exists, add the opts.new_edits value to the list of parameters.
      // If new_edits = false then the database will NOT assign this document a
      // new revision number
      if (opts && typeof opts.new_edits !== 'undefined') {
        params.push('new_edits=' + opts.new_edits);
      }

      // Format the list of parameters into a valid URI query string
      params = params.join('&');
      if (params !== '') {
        params = '?' + params;
      }

      var ajaxOpts = {
        method: 'PUT',
        url: genDBUrl(host, encodeDocId(doc._id)) + params,
        body: doc
      };

      return Promise.resolve().then(function () {
        var hasNonStubAttachments = doc._attachments &&
          Object.keys(doc._attachments).filter(function (att) {
            return !doc._attachments[att].stub;
          }).length;
        if (hasNonStubAttachments) {
          // use multipart/related for more efficient attachment uploading
          var multipart = createMultipart(doc);
          ajaxOpts.body = multipart.body;
          ajaxOpts.processData = false;
          ajaxOpts.headers = multipart.headers;
        }
      }).catch(function () {
        throw new Error('Did you forget to base64-encode an attachment?');
      }).then(function () {
        return ajaxPromise(opts, ajaxOpts);
      }).then(function (res) {
        res.ok = true; // smooths out cloudant not doing this
        callback(null, res);
      });
    }).catch(callback);
  });

  // Add the document given by doc (in JSON string format) to the database
  // given by host. This does not assume that doc is a new document
  // (i.e. does not have a _id or a _rev field.)
  api.post = adapterFun('post', function (doc, opts, callback) {
    if (typeof doc !== 'object') {
      return callback(errors.error(errors.NOT_AN_OBJECT));
    }
    if (! ("_id" in doc)) {
      doc._id = uuid();
    }
    api.put(doc, opts, function (err, res) {
      if (err) {
        return callback(err);
      }
      res.ok = true;
      callback(null, res);
    });
  });

  // Update/create multiple documents given by req in the database
  // given by host.
  api._bulkDocs = function (req, opts, callback) {
    // If new_edits=false then it prevents the database from creating
    // new revision numbers for the documents. Instead it just uses
    // the old ones. This is used in database replication.
    req.new_edits = opts.new_edits;

    setup().then(function () {
      return Promise.all(req.docs.map(preprocessAttachments));
    }).then(function () {
      // Update/create the documents
      ajax(opts, {
        method: 'POST',
        url: genDBUrl(host, '_bulk_docs'),
        body: req
      }, function (err, results) {
        if (err) {
          return callback(err);
        }
        results.forEach(function (result) {
          result.ok = true; // smooths out cloudant not adding this
        });
        callback(null, results);
      });
    }).catch(callback);
  };

  // Get a listing of the documents in the database given
  // by host and ordered by increasing id.
  api.allDocs = adapterFun('allDocs', function (opts, callback) {
    // List of parameters to add to the GET request
    var params = [];
    var body;
    var method = 'GET';

    if (opts.conflicts) {
      params.push('conflicts=true');
    }

    // If opts.descending is truthy add it to params
    if (opts.descending) {
      params.push('descending=true');
    }

    // If opts.include_docs exists, add the include_docs value to the
    // list of parameters.
    // If include_docs=true then include the associated document with each
    // result.
    if (opts.include_docs) {
      params.push('include_docs=true');
    }

    if (opts.attachments) {
      // added in CouchDB 1.6.0
      params.push('attachments=true');
    }

    if (opts.key) {
      params.push('key=' + encodeURIComponent(JSON.stringify(opts.key)));
    }

    if (opts.start_key) {
      opts.startkey = opts.start_key;
    }

    // If opts.startkey exists, add the startkey value to the list of
    // parameters.
    // If startkey is given then the returned list of documents will
    // start with the document whose id is startkey.
    if (opts.startkey) {
      params.push('startkey=' +
        encodeURIComponent(JSON.stringify(opts.startkey)));
    }

    if (opts.end_key) {
      opts.endkey = opts.end_key;
    }

    // If opts.endkey exists, add the endkey value to the list of parameters.
    // If endkey is given then the returned list of docuemnts will
    // end with the document whose id is endkey.
    if (opts.endkey) {
      params.push('endkey=' + encodeURIComponent(JSON.stringify(opts.endkey)));
    }

    if (typeof opts.inclusive_end !== 'undefined') {
      params.push('inclusive_end=' + !!opts.inclusive_end);
    }

    // If opts.limit exists, add the limit value to the parameter list.
    if (typeof opts.limit !== 'undefined') {
      params.push('limit=' + opts.limit);
    }

    if (typeof opts.skip !== 'undefined') {
      params.push('skip=' + opts.skip);
    }

    // Format the list of parameters into a valid URI query string
    params = params.join('&');
    if (params !== '') {
      params = '?' + params;
    }

    if (typeof opts.keys !== 'undefined') {

      var keysAsString =
        'keys=' + encodeURIComponent(JSON.stringify(opts.keys));
      if (keysAsString.length + params.length + 1 <= MAX_URL_LENGTH) {
        // If the keys are short enough, do a GET. we do this to work around
        // Safari not understanding 304s on POSTs (see issue #1239)
        params += (params.indexOf('?') !== -1 ? '&' : '?') + keysAsString;
      } else {
        // If keys are too long, issue a POST request to circumvent GET
        // query string limits
        // see http://wiki.apache.org/couchdb/HTTP_view_API#Querying_Options
        method = 'POST';
        body = {keys: opts.keys};
      }
    }

    // Get the document listing
    ajaxPromise(opts, {
      method: method,
      url: genDBUrl(host, '_all_docs' + params),
      body: body
    }).then(function (res) {
      if (opts.include_docs && opts.attachments && opts.binary) {
        res.rows.forEach(readAttachmentsAsBlobOrBuffer);
      }
      callback(null, res);
    }).catch(callback);
  });

  // Get a list of changes made to documents in the database given by host.
  // TODO According to the README, there should be two other methods here,
  // api.changes.addListener and api.changes.removeListener.
  api._changes = function (opts) {

    // We internally page the results of a changes request, this means
    // if there is a large set of changes to be returned we can start
    // processing them quicker instead of waiting on the entire
    // set of changes to return and attempting to process them at once
    var batchSize = 'batch_size' in opts ? opts.batch_size : CHANGES_BATCH_SIZE;

    opts = clone(opts);
    opts.timeout = opts.timeout || ajaxOpts.timeout || 30 * 1000;

    // We give a 5 second buffer for CouchDB changes to respond with
    // an ok timeout
    var params = { timeout: opts.timeout - (5 * 1000) };
    var limit = (typeof opts.limit !== 'undefined') ? opts.limit : false;
    var returnDocs;
    if ('returnDocs' in opts) {
      returnDocs = opts.returnDocs;
    } else {
      returnDocs = true;
    }
    //
    var leftToFetch = limit;

    if (opts.style) {
      params.style = opts.style;
    }

    if (opts.include_docs || opts.filter && typeof opts.filter === 'function') {
      params.include_docs = true;
    }

    if (opts.attachments) {
      params.attachments = true;
    }

    if (opts.continuous) {
      params.feed = 'longpoll';
    }

    if (opts.conflicts) {
      params.conflicts = true;
    }

    if (opts.descending) {
      params.descending = true;
    }

    // Default hearbeat to 10 seconds
    params.heartbeat = opts.heartbeat || 10000;

    if (opts.filter && typeof opts.filter === 'string') {
      params.filter = opts.filter;
      if (opts.filter === '_view' &&
          opts.view &&
          typeof opts.view === 'string') {
        params.view = opts.view;
      }
    }

    // If opts.query_params exists, pass it through to the changes request.
    // These parameters may be used by the filter on the source database.
    if (opts.query_params && typeof opts.query_params === 'object') {
      for (var param_name in opts.query_params) {
        /* istanbul ignore else */
        if (opts.query_params.hasOwnProperty(param_name)) {
          params[param_name] = opts.query_params[param_name];
        }
      }
    }

    var method = 'GET';
    var body;

    if (opts.doc_ids) {
      // set this automagically for the user; it's annoying that couchdb
      // requires both a "filter" and a "doc_ids" param.
      params.filter = '_doc_ids';

      var docIdsJson = JSON.stringify(opts.doc_ids);

      if (docIdsJson.length < MAX_URL_LENGTH) {
        params.doc_ids = docIdsJson;
      } else {
        // anything greater than ~2000 is unsafe for gets, so
        // use POST instead
        method = 'POST';
        body = {doc_ids: opts.doc_ids };
      }
    }

    var xhr;
    var lastFetchedSeq;

    // Get all the changes starting wtih the one immediately after the
    // sequence number given by since.
    var fetch = function (since, callback) {
      if (opts.aborted) {
        return;
      }
      params.since = since;
      // "since" can be any kind of json object in Coudant/CouchDB 2.x
      /* istanbul ignore next */
      if (typeof params.since === "object") {
        params.since = JSON.stringify(params.since);
      }

      if (opts.descending) {
        if (limit) {
          params.limit = leftToFetch;
        }
      } else {
        params.limit = (!limit || leftToFetch > batchSize) ?
          batchSize : leftToFetch;
      }

      var paramStr = '?' + Object.keys(params).map(function (k) {
        return k + '=' + encodeURIComponent(params[k]);
      }).join('&');

      // Set the options for the ajax call
      var xhrOpts = {
        method: method,
        url: genDBUrl(host, '_changes' + paramStr),
        // _changes can take a long time to generate, especially when filtered
        timeout: opts.timeout,
        body: body
      };
      lastFetchedSeq = since;

      /* istanbul ignore if */
      if (opts.aborted) {
        return;
      }

      // Get the changes
      setup().then(function() {
        xhr = ajax(opts, xhrOpts, callback);
      }).catch(callback);
    };

    // If opts.since exists, get all the changes from the sequence
    // number given by opts.since. Otherwise, get all the changes
    // from the sequence number 0.
    var results = {results: []};

    var fetched = function (err, res) {
      if (opts.aborted) {
        return;
      }
      var raw_results_length = 0;
      // If the result of the ajax call (res) contains changes (res.results)
      if (res && res.results) {
        raw_results_length = res.results.length;
        results.last_seq = res.last_seq;
        // For each change
        var req = {};
        req.query = opts.query_params;
        res.results = res.results.filter(function (c) {
          leftToFetch--;
          var ret = filterChange(opts)(c);
          if (ret) {
            if (opts.include_docs && opts.attachments && opts.binary) {
              readAttachmentsAsBlobOrBuffer(c);
            }
            if (returnDocs) {
              results.results.push(c);
            }
            opts.onChange(c);
          }
          return ret;
        });
      } else if (err) {
        // In case of an error, stop listening for changes and call
        // opts.complete
        opts.aborted = true;
        opts.complete(err);
        return;
      }

      // The changes feed may have timed out with no results
      // if so reuse last update sequence
      if (res && res.last_seq) {
        lastFetchedSeq = res.last_seq;
      }

      var finished = (limit && leftToFetch <= 0) ||
        (res && raw_results_length < batchSize) ||
        (opts.descending);

      if ((opts.continuous && !(limit && leftToFetch <= 0)) || !finished) {
        // Queue a call to fetch again with the newest sequence number
        setTimeout(function () { fetch(lastFetchedSeq, fetched); }, 0);
      } else {
        // We're done, call the callback
        opts.complete(null, results);
      }
    };

    fetch(opts.since || 0, fetched);

    // Return a method to cancel this method from processing any more
    return {
      cancel: function () {
        opts.aborted = true;
        if (xhr) {
          xhr.abort();
        }
      }
    };
  };

  // Given a set of document/revision IDs (given by req), tets the subset of
  // those that do NOT correspond to revisions stored in the database.
  // See http://wiki.apache.org/couchdb/HttpPostRevsDiff
  api.revsDiff = adapterFun('revsDiff', function (req, opts, callback) {
    // Get the missing document/revision IDs
    ajax(opts, {
      method: 'POST',
      url: genDBUrl(host, '_revs_diff'),
      body: req
    }, callback);
  });

  api._close = function (callback) {
    callback();
  };

  api._destroy = adapterFun('destroy', function (options, callback) {
    setup().then(function() {
      ajax(options, {
        url: genDBUrl(host, ''),
        method: 'DELETE'
      }, function (err, resp) {
        /* istanbul ignore next */
        if (err) {
          api.emit('error', err);
          return callback(err);
        }
        api.emit('destroyed');
        api.constructor.emit('destroyed', opts.name);
        callback(null, resp);
      });
    }).catch(callback);
  });
}

// HttpPouch is a valid adapter.
HttpPouch.valid = function () {
  return true;
};

module.exports = HttpPouch;