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| <h1 class="page">Distributed James Server — Consistency Model</h1> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>This page presents the consistency model used by the Distributed Server and |
| points to the tools built around it.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_data_replication"><a class="anchor" href="#_data_replication"></a>Data Replication</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The Distributed Server relies on different storage technologies, all having their own |
| consistency models.</p> |
| </div> |
| <div class="paragraph"> |
| <p>These data stores replicate data in order to enforce some level of availability. We call |
| this process replication. By consistency, we mean the ability for all replica to hold the |
| same data. By availability, we mean the ability for a replica to answer a request.</p> |
| </div> |
| <div class="paragraph"> |
| <p>In distributed systems, <a href="https://en.wikipedia.org/wiki/CAP_theorem">according to the CAP theorem</a>, |
| as we will necessarily encounter network partitions, then trade-offs need to be made between |
| consistency and availability.</p> |
| </div> |
| <div class="paragraph"> |
| <p>This section details this trade-off for data stores used by the Distributed Server.</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_cassandra_consistency_model"><a class="anchor" href="#_cassandra_consistency_model"></a>Cassandra consistency model</h3> |
| <div class="paragraph"> |
| <p><a href="https://cassandra.apache.org/">Cassandra</a> is an |
| <a href="https://en.wikipedia.org/wiki/Eventual_consistency">eventually consistent</a> data store. |
| This means that replica can hold diverging data, but are guaranteed to converge over time.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Several mechanisms are built in Cassandra to enforce this convergence, and need to be |
| leveraged by <strong>Distributed Server Administrator</strong>. Namely |
| <a href="https://docs.datastax.com/en/dse/5.1/dse-admin/datastax_enterprise/tools/nodetool/toolsRepair.html">nodetool repair</a>, |
| <a href="https://cassandra.apache.org/doc/latest/operating/hints.html">Hinted hand-off</a> and |
| <a href="https://cassandra.apache.org/doc/latest/operating/read_repair.html">Read repair</a>.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The Distributed Server tries to mitigate inconsistencies by relying on |
| <a href="https://docs.datastax.com/en/archived/cassandra/3.0/cassandra/dml/dmlConfigConsistency.html">QUORUM</a> read and write levels. |
| This means that a majority of replica are needed for read and write operations to be performed.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Critical business operations, like UID allocation, rely on strong consistency mechanisms brought by |
| <a href="https://www.datastax.com/blog/2013/07/lightweight-transactions-cassandra-20">lightweight transaction</a>.</p> |
| </div> |
| <div class="sect3"> |
| <h4 id="_about_multi_data_center_setups"><a class="anchor" href="#_about_multi_data_center_setups"></a>About multi data-center setups</h4> |
| <div class="paragraph"> |
| <p>As strong consistency is required for some operations, and as lightweight transactions are |
| slow across data centers, running James with a |
| <a href="https://docs.datastax.com/en/ddac/doc/datastax_enterprise/production/DDACmultiDCperWorkloadType.html">multi data-center</a> |
| Cassandra setup is discouraged.</p> |
| </div> |
| <div class="paragraph"> |
| <p>However <a href="../configure/cassandra.html" class="xref page">this page</a> enables setting alternative read level, |
| which could be acceptable regarding limited requirements. <code>LOCAL_QUORUM</code> coupled with <code>LOCAL_SERIAL</code> |
| is likely the only scalable setup.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Running the Distributed Server IMAP server in a multi datacenter setup will likely result either in data loss, |
| or very slow operations - as we rely on monotic UID generation, without strong consistency, UIDs could be allocated |
| several times. Other protocols, like JMAP, suffer less from such limitations.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_elasticsearch_consistency_model"><a class="anchor" href="#_elasticsearch_consistency_model"></a>ElasticSearch consistency model</h3> |
| <div class="paragraph"> |
| <p>ElasticSearch relies on <a href="https://www.elastic.co/blog/a-new-era-for-cluster-coordination-in-elasticsearch">strong consistency</a> |
| with home grown algorithm.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The 6.x release line, that the distributed server is using is known to be slow to recover from failures.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Be aware that data is asynchronously indexed in ElasticSearch, changes will be eventually visible.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_rabbitmq_consistency_model"><a class="anchor" href="#_rabbitmq_consistency_model"></a>RabbitMQ consistency model</h3> |
| <div class="paragraph"> |
| <p>The Distributed Server relies out of the box on a single RabbitMQ server, thus consistency concerns |
| are not (yet) applicable. Availability concerns are applicable.</p> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_denormalization"><a class="anchor" href="#_denormalization"></a>Denormalization</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>In Cassandra, data needs to be structured to match the read patterns. To support several conflicting |
| read patterns, the data needs to be duplicated into different structures. This process is called |
| denormalization.</p> |
| </div> |
| <div class="paragraph"> |
| <p>While data can be consistent at the table level, some inconsistencies can sneak in at the applicative |
| level across denormalization tables.</p> |
| </div> |
| <div class="paragraph"> |
| <p>We write to a "table of truth" first, then duplicate the data to denormalization tables.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The Distributed server offers several mechanisms to mitigate these inconsistencies:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Writes to denormalization tables are retried.</p> |
| </li> |
| <li> |
| <p>Some <a href="../operate/guide.html#_solving_cassandra_inconsistencies" class="xref page">SolveInconsistencies tasks</a> are exposed and are able to heal a given denormalization table. |
| They reset the "deduplication tables" content to the "table of truth" content.</p> |
| </li> |
| <li> |
| <p><a href="https://github.com/apache/james-project/blob/master/src/adr/0042-applicative-read-repairs.md">Read repairs</a>, |
| when implemented for a given denormalization, enables auto-healing. When an inconsistency is detected, They reset the |
| "deduplication tables" entry to the "table of truth" entry.</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_consistency_across_data_stores"><a class="anchor" href="#_consistency_across_data_stores"></a>Consistency across data stores</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The Distributed Server leverages several data stores:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Cassandra is used for metadata storage</p> |
| </li> |
| <li> |
| <p>ElasticSearch for search</p> |
| </li> |
| <li> |
| <p>Object Storage for large object storage</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Thus the Distributed Server also offers mechanisms to enforce consistency across data stores.</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_write_path_organisation"><a class="anchor" href="#_write_path_organisation"></a>Write path organisation</h3> |
| <div class="paragraph"> |
| <p>The primary data stores are composed of Cassandra for metadata and Object storage for binary data.</p> |
| </div> |
| <div class="paragraph"> |
| <p>To ensure the data referenced in Cassandra is pointing to a valid object in the object store, we write |
| the object store payload first, then write the corresponding metadata in Cassandra.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Such a procedure avoids metadata pointing to unexisting blobs, however might lead to some unreferenced |
| blobs.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_cassandra_elasticsearch"><a class="anchor" href="#_cassandra_elasticsearch"></a>Cassandra <⇒ ElasticSearch</h3> |
| <div class="paragraph"> |
| <p>After being written to the primary stores (namely Cassandra & Object Storage), email content is |
| asynchronously indexed into ElasticSearch.</p> |
| </div> |
| <div class="paragraph"> |
| <p>This process, called the EventBus, which retries temporary errors, and stores transient errors for |
| later admin-triggered retries is described further <a href="../operate/guide.html#_mailbox_event_bus" class="xref page">here</a>. |
| His role is to spread load and limit inconsistencies.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Furthermore, some <a href="../operate/guide.html#_usual_troubleshooting_procedures" class="xref page">re-indexing tasks</a> |
| enables to re-synchronise ElasticSearch content with the primary data stores</p> |
| </div> |
| </div> |
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