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| <div class="section" id="global-replicated-log"> |
| <h1>Global Replicated Log<a class="headerlink" href="#global-replicated-log" title="Permalink to this headline">¶</a></h1> |
| <p>A typical setup for DistributedLog is within a datacenter. But a global setup is required for |
| providing global replicated logs for distributed key/value store to achieve strong consistency |
| across multiple datacenters. <cite>Global</cite> here means across datacenters, which is different from |
| <cite>Local</cite> meaning within a datacenter.</p> |
| <p>A global setup of DistributedLog is organized as a set of <cite>regions</cite>, where each region is the |
| rough analog of a local setup. Regions are the unit of administrative deployment. The set of |
| regions is also the set of locations across which data can be replicated. Regions can be added |
| to or removed from a running system as new datacenters are brought into service and old ones |
| are turned off, respectively. Regions are also the unit of physical isolation: there may be one |
| or more regions in a datacenter if they have isolated power or network supplies.</p> |
| <div class="figure align-center"> |
| <img alt="../_images/globalreplicatedlog.png" src="../_images/globalreplicatedlog.png" /> |
| <p class="caption">Figure 1. Global Replicated Log</p> |
| </div> |
| <p>Figure 1 illustrates the servers in a <cite>Global Replicated Log</cite> setup. There is no inter datacenter |
| communication between write proxies or log segment storage nodes. The only component that does |
| inter datacenters communications within its hosts is the “Global” metadata store, which is a global |
| setup of ZooKeeper. Write clients will talk to the write proxies in its local region to bootstrap |
| the ownership cache and redirect to correct write proxies in other regions through direct TCP |
| connections. While readers will identify the regions of the log segment storage nodes according to |
| the <cite>region aware</cite> placement policy, and try reading from local region at most of the time and |
| speculatively try on remote regions.</p> |
| <div class="section" id="region-aware-data-placement-policy"> |
| <h2>Region Aware Data Placement Policy<a class="headerlink" href="#region-aware-data-placement-policy" title="Permalink to this headline">¶</a></h2> |
| <p>Region aware placement policy uses hierarchical allocation where-in nodes are allocated so that data |
| is spread uniformly across the available regions and within each region it uses the <cite>rack-aware</cite> |
| placement policy to spread the data uniformly across the available racks.</p> |
| <p>Region aware placement policy is governed by a parameter ensures that the ack quorum covers at least |
| <em>minRegionsForDurability</em> distinct regions. This ensures that the system can survive the failure of |
| <cite>(totalRegions - minRegionsForDurability)</cite> regions without loss of availability. For example if we |
| have bookie nodes in <em>5</em> regions and if the <em>minRegionsForDurability</em> is <em>3</em> then we can survive the |
| failure of <cite>(5 - 3) = 2</cite> regions.</p> |
| <p>The placement algorithm follows the following simple invariant:</p> |
| <div class="highlight-python"><pre>There is no combination of nodes that would satisfy the ack quorum with |
| less than "minRegionsForDurability" responses.</pre> |
| <div style='display:none;' class='raw-code'><pre>There is no combination of nodes that would satisfy the ack quorum with |
| less than "minRegionsForDurability" responses.</pre> |
| </div></div> |
| <p>This invariant ensures that enforcing ack quorum is sufficient to enforce that the entry has been made durable |
| in <em>minRegionsForDurability</em> regions.</p> |
| <p>The <em>minRegionsForDurability</em> requirement enforces constraints on the minimum ack quorum as we want to ensure |
| that when we run in degraded mode - <em>i.e. when only a subset of the regions are available</em> - we would still not |
| be able to allocate nodes in such a way that the ack quorum would be satisfied by fewer than <em>minRegionsForDurability</em> |
| regions.</p> |
| <p>For instance consider the following scenario with three regions each containing 20 bookie nodes:</p> |
| <div class="highlight-python"><pre>minRegionsForDurability = 2 |
| ensemble size = write quorum = 15 |
| ack quorum = 8</pre> |
| <div style='display:none;' class='raw-code'><pre>minRegionsForDurability = 2 |
| ensemble size = write quorum = 15 |
| ack quorum = 8</pre> |
| </div></div> |
| <p>Let’s say that one of the regions is currently unavailable and we want to still ensure that writes can continue. |
| The ensemble placement may then have to choose bookies from the two available regions. Given that <em>15</em> bookies have |
| to be allocated, we will have to allocate at least <em>8</em> bookies from one of the remaining regions - but with ack quorum |
| of <em>8</em> we run the risk of satisfying ack quorum with bookies from a single region. Therefore we must require that |
| the ack quorum is greater than <em>8</em>.</p> |
| </div> |
| <div class="section" id="cross-region-speculative-reads"> |
| <h2>Cross Region Speculative Reads<a class="headerlink" href="#cross-region-speculative-reads" title="Permalink to this headline">¶</a></h2> |
| <p>As discussed before, read requests can be satisfied by any replica of the data, however for high availability |
| speculative requests are sent to multiple copies to ensure that at least one of the requests returns within |
| the time specified by the <em>SLA</em>. The reader consults the data placement policy to get the list of replicas that |
| can satisfy the request in the order of preference. This order is decided as follows:</p> |
| <ul class="simple"> |
| <li>The first node in the list is always the bookie node that is closest to the client - if more than one such nodes exist, one is chosen at random.</li> |
| <li>The second node is usually the closest node in a different failure domain. In the case of a two level hierarchy that would be a node in a different rack.</li> |
| <li>The third node is chosen from a different region</li> |
| </ul> |
| <p>The delay between successive speculative read requests ensures that the probability of sending the <em>nth</em> |
| speculative read request decays exponentially with <em>n</em>. This ensures that the number of requests that go to |
| farther nodes is still kept at a minimum. However by making sure that we cross failure domains in the first |
| few speculative requests improves fault-tolerance of the reader. Transient node failures are transparently |
| handled by the reader by this simple and generalizable speculative read policy. This can be thought of as |
| the most granular form of failover where each request essentially fails-over to an alternate node if the |
| primary node it attempted to access is unavailable. In practice we have found this to also better handle |
| network congestion where routes between specific pairs of nodes may become unavailable without necessarily |
| making the nodes completely inaccessible.</p> |
| <p>In addition to static decisions based on the location of the bookie nodes, we can also make dynamic decisions |
| based on observed latency or failure rates from specific bookies. These statistics are tracked by the bookie |
| client and are used to influence the order in which speculative read requests are scheduled. This again is |
| able to capture partial network outages that affect specific routes within the network.</p> |
| </div> |
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| <li class="toctree-l1 current"><a class="current reference internal" href="">Global Replicated Log</a><ul> |
| <li class="toctree-l2"><a class="reference internal" href="#region-aware-data-placement-policy">Region Aware Data Placement Policy</a></li> |
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| // xmlhttp=new XMLHttpRequest(); |
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| // var event_data = { |
| // comment: '', // see todo note below |
| // rating: data.rating, |
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| // xmlhttp.open("GET", "/event/distributedlog/rating?" + $.param(event_data), false); |
| // xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded"); |
| // // todo: implement comment form in rating slide out, |
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