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| <main class="article default-main full-800" data-ceiling="topbar"> |
| <div class="article-banner"> |
| <p>You are viewing the documentation for a prerelease version.</p> |
| </div> |
| <div class="article-header"> |
| <nav class="crumbs" aria-label="breadcrumbs"> |
| <ul> |
| <li class="crumb">Cassandra</li> |
| <li class="crumb"><a href="../index.html">Managing</a></li> |
| <li class="crumb"><a href="index.html">Operating</a></li> |
| <li class="crumb"><a href="read_repair.html">Read repair</a></li> |
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| <article class="doc"> |
| <h1 class="page">Read repair</h1> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Read Repair is the process of repairing data replicas during a read |
| request. If all replicas involved in a read request at the given read |
| consistency level are consistent the data is returned to the client and |
| no read repair is needed. But if the replicas involved in a read request |
| at the given consistency level are not consistent a read repair is |
| performed to make replicas involved in the read request consistent. The |
| most up-to-date data is returned to the client. The read repair runs in |
| the foreground and is blocking in that a response is not returned to the |
| client until the read repair has completed and up-to-date data is |
| constructed.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="expectation-of-monotonic-quorum-reads"><a class="anchor" href="#expectation-of-monotonic-quorum-reads"></a>Expectation of Monotonic Quorum Reads</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Cassandra uses a blocking read repair to ensure the expectation of |
| "monotonic quorum reads" i.e. that in 2 successive quorum reads, it’s |
| guaranteed the 2nd one won’t get something older than the 1st one, and |
| this even if a failed quorum write made a write of the most up to date |
| value only to a minority of replicas. "Quorum" means majority of nodes |
| among replicas.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="table-level-configuration-of-monotonic-reads"><a class="anchor" href="#table-level-configuration-of-monotonic-reads"></a>Table level configuration of monotonic reads</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Cassandra 4.0 adds support for table level configuration of monotonic |
| reads |
| (<a href="https://issues.apache.org/jira/browse/CASSANDRA-14635">CASSANDRA-14635</a>). |
| The <code>read_repair</code> table option has been added to table schema, with the |
| options <code>blocking</code> (default), and <code>none</code>.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The <code>read_repair</code> option configures the read repair behavior to allow |
| tuning for various performance and consistency behaviors. Two |
| consistency properties are affected by read repair behavior.</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Monotonic Quorum Reads: Provided by <code>BLOCKING</code>. Monotonic quorum reads |
| prevents reads from appearing to go back in time in some circumstances. |
| When monotonic quorum reads are not provided and a write fails to reach |
| a quorum of replicas, it may be visible in one read, and then disappear |
| in a subsequent read.</p> |
| </li> |
| <li> |
| <p>Write Atomicity: Provided by <code>NONE</code>. Write atomicity prevents reads |
| from returning partially applied writes. Cassandra attempts to provide |
| partition level write atomicity, but since only the data covered by a |
| <code>SELECT</code> statement is repaired by a read repair, read repair can break |
| write atomicity when data is read at a more granular level than it is |
| written. For example read repair can break write atomicity if you write |
| multiple rows to a clustered partition in a batch, but then select a |
| single row by specifying the clustering column in a <code>SELECT</code> statement.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>The available read repair settings are:</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="blocking"><a class="anchor" href="#blocking"></a>Blocking</h3> |
| <div class="paragraph"> |
| <p>The default setting. When <code>read_repair</code> is set to <code>BLOCKING</code>, and a read |
| repair is started, the read will block on writes sent to other replicas |
| until the CL is reached by the writes. Provides monotonic quorum reads, |
| but not partition level write atomicity.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="none"><a class="anchor" href="#none"></a>None</h3> |
| <div class="paragraph"> |
| <p>When <code>read_repair</code> is set to <code>NONE</code>, the coordinator will reconcile any |
| differences between replicas, but will not attempt to repair them. |
| Provides partition level write atomicity, but not monotonic quorum |
| reads.</p> |
| </div> |
| <div class="paragraph"> |
| <p>An example of using the <code>NONE</code> setting for the <code>read_repair</code> option is |
| as follows:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-none hljs" data-lang="none">CREATE TABLE ks.tbl (k INT, c INT, v INT, PRIMARY KEY (k,c)) with read_repair='NONE'");</code></pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="read-repair-example"><a class="anchor" href="#read-repair-example"></a>Read Repair Example</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>To illustrate read repair with an example, consider that a client sends |
| a read request with read consistency level <code>TWO</code> to a 5-node cluster as |
| illustrated in Figure 1. Read consistency level determines how many |
| replica nodes must return a response before the read request is |
| considered successful.</p> |
| </div> |
| <div class="imageblock"> |
| <div class="content"> |
| <img src="../../_images/Figure_1_read_repair.jpg" alt="image"> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Figure 1. Client sends read request to a 5-node Cluster</p> |
| </div> |
| <div class="paragraph"> |
| <p>Three nodes host replicas for the requested data as illustrated in |
| Figure 2. With a read consistency level of <code>TWO</code> two replica nodes must |
| return a response for the read request to be considered successful. If |
| the node the client sends request to hosts a replica of the data |
| requested only one other replica node needs to be sent a read request |
| to. But if the receiving node does not host a replica for the requested |
| data the node becomes a coordinator node and forwards the read request |
| to a node that hosts a replica. A direct read request is forwarded to |
| the fastest node (as determined by dynamic snitch) as shown in Figure 2. |
| A direct read request is a full read and returns the requested data.</p> |
| </div> |
| <div class="imageblock"> |
| <div class="content"> |
| <img src="../../_images/Figure_2_read_repair.jpg" alt="image"> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Figure 2. Direct Read Request sent to Fastest Replica Node</p> |
| </div> |
| <div class="paragraph"> |
| <p>Next, the coordinator node sends the requisite number of additional |
| requests to satisfy the consistency level, which is <code>TWO</code>. The |
| coordinator node needs to send one more read request for a total of two. |
| All read requests additional to the first direct read request are digest |
| read requests. A digest read request is not a full read and only returns |
| the hash value of the data. Only a hash value is returned to reduce the |
| network data traffic. In the example being discussed the coordinator |
| node sends one digest read request to a node hosting a replica as |
| illustrated in Figure 3.</p> |
| </div> |
| <div class="imageblock"> |
| <div class="content"> |
| <img src="../../_images/Figure_3_read_repair.jpg" alt="image"> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Figure 3. Coordinator Sends a Digest Read Request</p> |
| </div> |
| <div class="paragraph"> |
| <p>The coordinator node has received a full copy of data from one node and |
| a hash value for the data from another node. To compare the data |
| returned a hash value is calculated for the full copy of data. The two |
| hash values are compared. If the hash values are the same no read repair |
| is needed and the full copy of requested data is returned to the client. |
| The coordinator node only performed a total of two replica read request |
| because the read consistency level is <code>TWO</code> in the example. If the |
| consistency level were higher such as <code>THREE</code>, three replica nodes would |
| need to respond to a read request and only if all digest or hash values |
| were to match with the hash value of the full copy of data would the |
| read request be considered successful and the data returned to the |
| client.</p> |
| </div> |
| <div class="paragraph"> |
| <p>But, if the hash value/s from the digest read request/s are not the same |
| as the hash value of the data from the full read request of the first |
| replica node it implies that an inconsistency in the replicas exists. To |
| fix the inconsistency a read repair is performed.</p> |
| </div> |
| <div class="paragraph"> |
| <p>For example, consider that that digest request returns a hash value that |
| is not the same as the hash value of the data from the direct full read |
| request. We would need to make the replicas consistent for which the |
| coordinator node sends a direct (full) read request to the replica node |
| that it sent a digest read request to earlier as illustrated in Figure |
| 4.</p> |
| </div> |
| <div class="imageblock"> |
| <div class="content"> |
| <img src="../../_images/Figure_4_read_repair.jpg" alt="image"> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Figure 4. Coordinator sends Direct Read Request to Replica Node it had |
| sent Digest Read Request to</p> |
| </div> |
| <div class="paragraph"> |
| <p>After receiving the data from the second replica node the coordinator |
| has data from two of the replica nodes. It only needs two replicas as |
| the read consistency level is <code>TWO</code> in the example. Data from the two |
| replicas is compared and based on the timestamps the most recent replica |
| is selected. Data may need to be merged to construct an up-to-date copy |
| of data if one replica has data for only some of the columns. In the |
| example, if the data from the first direct read request is found to be |
| outdated and the data from the second full read request to be the latest |
| read, repair needs to be performed on Replica 2. If a new up-to-date |
| data is constructed by merging the two replicas a read repair would be |
| needed on both the replicas involved. For example, a read repair is |
| performed on Replica 2 as illustrated in Figure 5.</p> |
| </div> |
| <div class="imageblock"> |
| <div class="content"> |
| <img src="../../_images/Figure_5_read_repair.jpg" alt="image"> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Figure 5. Coordinator performs Read Repair</p> |
| </div> |
| <div class="paragraph"> |
| <p>The most up-to-date data is returned to the client as illustrated in |
| Figure 6. From the three replicas Replica 1 is not even read and thus |
| not repaired. Replica 2 is repaired. Replica 3 is the most up-to-date |
| and returned to client.</p> |
| </div> |
| <div class="imageblock"> |
| <div class="content"> |
| <img src="../../_images/Figure_6_read_repair.jpg" alt="image"> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Figure 6. Most up-to-date Data returned to Client</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="read-consistency-level-and-read-repair"><a class="anchor" href="#read-consistency-level-and-read-repair"></a>Read Consistency Level and Read Repair</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The read consistency is most significant in determining if a read repair |
| needs to be performed. As discussed in Table 1 a read repair is not |
| needed for all of the consistency levels.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Table 1. Read Repair based on Read Consistency Level</p> |
| </div> |
| <table class="tableblock frame-all grid-all" style="width: 93%;"> |
| <colgroup> |
| <col style="width: 35%;"> |
| <col style="width: 65%;"> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">Read Consistency Level</th> |
| <th class="tableblock halign-left valign-top">Description</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">ONE</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Read repair is not performed as the data from the first direct |
| read request satisfies the consistency level ONE. No digest read |
| requests are involved for finding mismatches in data.</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">TWO</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Read repair is performed if inconsistencies in data are found as |
| determined by the direct and digest read requests.</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">THREE</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Read repair is performed if inconsistencies in data are found as |
| determined by the direct and digest read requests.</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">LOCAL_ONE</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Read repair is not performed as the data from the direct |
| read request from the closest replica satisfies the consistency level |
| LOCAL_ONE.No digest read requests are involved for finding mismatches in |
| data.</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">LOCAL_QUORUM</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Read repair is performed if inconsistencies in data are |
| found as determined by the direct and digest read requests.</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">QUORUM</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">Read repair is performed if inconsistencies in data are found |
| as determined by the direct and digest read requests.</p></td> |
| </tr> |
| </tbody> |
| </table> |
| <div class="paragraph"> |
| <p>If read repair is performed it is made only on the replicas that are not |
| up-to-date and that are involved in the read request. The number of |
| replicas involved in a read request would be based on the read |
| consistency level; in the example it is two.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="improved-read-repair-blocking-behavior-in-cassandra-4-0"><a class="anchor" href="#improved-read-repair-blocking-behavior-in-cassandra-4-0"></a>Improved Read Repair Blocking Behavior in Cassandra 4.0</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Cassandra 4.0 makes two improvements to read repair blocking behavior |
| (<a href="https://issues.apache.org/jira/browse/CASSANDRA-10726">CASSANDRA-10726</a>).</p> |
| </div> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p>Speculative Retry of Full Data Read Requests. Cassandra 4.0 makes use |
| of speculative retry in sending read requests (full, not digest) to |
| replicas if a full data response is not received, whether in the initial |
| full read request or a full data read request during read repair. With |
| speculative retry if it looks like a response may not be received from |
| the initial set of replicas Cassandra sent messages to, to satisfy the |
| consistency level, it speculatively sends additional read request to |
| un-contacted replica/s. Cassandra 4.0 will also speculatively send a |
| repair mutation to a minority of nodes not involved in the read repair |
| data read / write cycle with the combined contents of all |
| un-acknowledged mutations if it looks like one may not respond. |
| Cassandra accepts acks from them in lieu of acks from the initial |
| mutations sent out, so long as it receives the same number of acks as |
| repair mutations transmitted.</p> |
| </li> |
| <li> |
| <p>Only blocks on Full Data Responses to satisfy the Consistency Level. |
| Cassandra 4.0 only blocks for what is needed for resolving the digest |
| mismatch and wait for enough full data responses to meet the consistency |
| level, no matter whether it’s speculative retry or read repair chance. |
| As an example, if it looks like Cassandra might not receive full data |
| requests from everyone in time, it sends additional requests to |
| additional replicas not contacted in the initial full data read. If the |
| collection of nodes that end up responding in time end up agreeing on |
| the data, the response from the disagreeing replica that started the |
| read repair is not considered, and won’t be included in the response to |
| the client, preserving the expectation of monotonic quorum reads.</p> |
| </li> |
| </ol> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="diagnostic-events-for-read-repairs"><a class="anchor" href="#diagnostic-events-for-read-repairs"></a>Diagnostic Events for Read Repairs</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Cassandra 4.0 adds diagnostic events for read repair |
| (<a href="https://issues.apache.org/jira/browse/CASSANDRA-14668">CASSANDRA-14668</a>) |
| that can be used for exposing information such as:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Contacted endpoints</p> |
| </li> |
| <li> |
| <p>Digest responses by endpoint</p> |
| </li> |
| <li> |
| <p>Affected partition keys</p> |
| </li> |
| <li> |
| <p>Speculated reads / writes</p> |
| </li> |
| <li> |
| <p>Update oversized</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="background-read-repair"><a class="anchor" href="#background-read-repair"></a>Background Read Repair</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Background read repair, which was configured using <code>read_repair_chance</code> |
| and <code>dclocal_read_repair_chance</code> settings in <code>cassandra.yaml</code> is removed |
| Cassandra 4.0 |
| (<a href="https://issues.apache.org/jira/browse/CASSANDRA-13910">CASSANDRA-13910</a>).</p> |
| </div> |
| <div class="paragraph"> |
| <p>Read repair is not an alternative for other kind of repairs such as full |
| repairs or replacing a node that keeps failing. The data returned even |
| after a read repair has been performed may not be the most up-to-date |
| data if consistency level is other than one requiring response from all |
| replicas.</p> |
| </div> |
| </div> |
| </div> |
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