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PHPLIB-790: Server selection tutorial and FAQ entry
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‎docs/faq.txt

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@@ -76,3 +76,60 @@ In addition to the aforementioned constants, these properties can also be
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inferred from :php:`phpinfo() <phpinfo>`. If your system has multiple PHP
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runtimes installed, double-check that you are examining the ``phpinfo()`` output
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for the correct environment.
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Server Selection Failures
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-------------------------
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The follow are all examples of
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:doc:`Server Selection </tutorial/server-selection>` failures:
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.. code-block:: none
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No suitable servers found (`serverSelectionTryOnce` set):
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[connection refused calling hello on 'a.example.com:27017']
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[connection refused calling hello on 'b.example.com:27017']
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No suitable servers found: `serverSelectionTimeoutMS` expired:
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[socket timeout calling hello on 'example.com:27017']
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No suitable servers found: `serverSelectionTimeoutMS` expired:
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[connection timeout calling hello on 'a.example.com:27017']
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[connection timeout calling hello on 'b.example.com:27017']
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[TLS handshake failed: -9806 calling hello on 'c.example.com:27017']
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No suitable servers found: `serverselectiontimeoutms` timed out:
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[TLS handshake failed: certificate verify failed (64): IP address mismatch calling hello on 'a.example.com:27017']
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[TLS handshake failed: certificate verify failed (64): IP address mismatch calling hello on 'b.example.com:27017']
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These errors typically manifest as a
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:php:`MongoDB\\Driver\\Exception\\ConnectionTimeoutException <mongodb-driver-exception-connectiontimeoutexception>`
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exception from the driver. The actual exception messages originate from
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libmongoc, which is the underlying library used by the PHP driver. Since these
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messages can take many forms, it's helpful to break down the structure of the
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message so you can better diagnose errors in your application.
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Messages will typically start with "No suitable servers found". The next part of
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the message indicates *how* server selection failed. By default, the PHP driver
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avoids a server selection loop and instead makes a single attempt (according to
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the ``serverSelectionTryOnce`` connection string option). If the driver is
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configured to utilize a loop, a message like "serverSelectionTimeoutMS expired"
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will tell us that we exhausted its time limit.
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The last component of the message tells us *why* server selection failed, and
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includes one or more errors directly from the topology scanner, which is the
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service responsible for connecting to and monitoring each host. Any host that
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last experienced an error during monitoring will be included in this list. These
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messages typically originate from low-level socket or TLS functions.
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The following is not meant to be exhaustive, but will hopefully point you in the
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right direction for analyzing the contributing factor(s) for a server selection
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failure:
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- "connection refused" likely indicates that the remote host is not listening on
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the expected port.
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- "connection timeout" could indicate a routing or firewall issue, or perhaps
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a timeout due to latency.
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- "socket timeout" suggests that a connection *was* established at some point
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but was dropped or otherwise timeout out due to latency.
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- "TLS handshake failed" suggests something related to TLS or OCSP verification
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and is sometimes indicative of misconfigured TLS certificates.

‎docs/tutorial.txt

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.. toctree::
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/tutorial/connecting
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/tutorial/server-selection
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/tutorial/crud
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/tutorial/collation
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/tutorial/commands

‎docs/tutorial/server-selection.txt

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===============================
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Server Selection and Monitoring
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===============================
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.. default-domain:: mongodb
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.. contents:: On this page
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:local:
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:backlinks: none
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:depth: 2
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:class: singlecol
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Server Selection and Monitoring
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-------------------------------
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Before any operation can be executed, the |php-library| must first select a
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server from the topology (e.g. replica set, sharded cluster). Selecting a server
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requires an accurate view of the topology, so the driver (i.e. ``mongodb``
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extension) regularly monitors the servers to which it is connected.
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In most other drivers, server discovery and monitoring is handled by a
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background thread; however, the PHP driver is single-threaded and must therefore
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perform monitoring *between* operations initiated by the application.
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Consider the following example application:
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.. code-block:: php
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<?php
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/**
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* When constructing a Client, the library creates a MongoDB\Driver\Manager
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* object from the driver. In turn, the driver will either create a libmongoc
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* client object (and persist it according to the constructor parameters) or
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* re-use a previously persisted client.
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*
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* Assuming a new libmongoc client was created, the host name(s) in the
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* connection string must be resolved via DNS. Likewise, if the connection
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* string includes a mongodb+srv scheme, SRV/TXT records must be resolved.
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* Following DNS resolution, the driver should then have a list of one or
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* more hosts to which it can connect. This is referred to as the seed list.
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*
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* If a previously persisted client was re-used, no DNS resolution is needed
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* and there will likely already be connections and topology state associated
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* with the client.
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*
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* Drivers perform no further IO when constructing a client, so control is
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* returned the the PHP script.
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*/
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$client = new MongoDB\Client('mongodb://a.example.com:27017/?replicaSet=rs0');
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/**
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* The library creates a MongoDB\Database object from the Client. This does
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* not entail any IO, as the Database and Collection objects only associate
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* a database or namespace with a Client object, respectively.
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*/
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$database = $client->test;
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/**
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* The library creates an internal object for this operation and must select
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* a server to use for executing that operation.
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*
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* If this is the first operation on the underlying libmongoc client, it must
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* first discover the topology. It does so by establishing connections to any
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* host(s) in the seed list (this may entail TLS and OCSP verification) and
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* issuing "hello" commands.
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*
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* In the case of a replica set, connecting to a single host in the seed list
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* should allow the driver to discover all other members in the replica set.
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* In the case of a sharded cluster, the driver will start with an initial
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* seed list of mongos hosts and, if SRV polling is utilized, may discover
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* additional mongos hosts over time.
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*
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* If the topology was already initialized (i.e. this is not the first
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* operation on the client), the driver may still need to perform monitoring
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* (i.e. "hello" commands) and refresh its view of the topology. This process
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* may entail adding or removing hosts from the topology.
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*
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* Once the topology has been discovered and any necessary monitoring has
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* been performed, the driver may select a server according to the rules
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* outlined in the server selection specification (e.g. applying a read
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* preference, filtering hosts by latency).
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*/
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$database->command(['ping' => 1]);
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Although the application consists of only a few lines of PHP, there is actually
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quite a lot going on behind the scenes! Interested readers can find this process
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discussed in greater detail in the following documents:
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- `Single-threaded Mode <http://mongoc.org/libmongoc/current/connection-pooling.html#single-mode>`_ in the libmongoc documentation
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- `Server Discovery and Monitoring <https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring.rst>`_ specification
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- `Server Selection <https://github.com/mongodb/specifications/blob/master/source/server-selection/server-selection.rst>`_ specification
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Connection String Options
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-------------------------
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There are several connection string options relevant to server selection and
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monitoring.
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connectTimeoutMS
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~~~~~~~~~~~~~~~~
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``connectTimeoutMS`` specifies the limit for both establishing a connection to
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a server *and* the socket timeout for server monitoring (``hello`` commands).
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This defaults to 10 seconds for single-threaded drivers such as PHP.
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When a server times out during monitoring, it will not be re-checked until at
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least five seconds
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(`cooldownMS <https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-monitoring.rst#cooldownms>`_)
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have elapsed. This timeout is intended to avoid having single-threaded drivers
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block for ``connectTimeoutMS`` on *each* subsequent scan after an error.
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Applications can consider setting this option to slightly more than the greatest
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latency among servers in the cluster. For example, if the greatest ``ping`` time
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between the PHP application server and a database server is 200ms, it may be
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reasonable to specify a timeout of one second. This would allow ample time for
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establishing a connection and monitoring an accessible server, while also
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significantly reducing the time to detect an inaccessible server.
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heartbeatFrequencyMS
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~~~~~~~~~~~~~~~~~~~~
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``heartbeatFrequencyMS`` determines how often monitoring should occur. This
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defaults to 60 seconds for single-threaded drivers and can be set as low as
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500ms.
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serverSelectionTimeoutMS
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~~~~~~~~~~~~~~~~~~~~~~~~
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``serverSelectionTimeoutMS`` determines the maximum amount of time to spend in
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the server selection loop. This defaults to 30 seconds, but applications will
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typically fail sooner if ``serverSelectionTryOnce`` is ``true`` and a smaller
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``connectTimeoutMS`` value is in effect.
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The original default was established at a time when replica set elections took
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much longer to complete. Applications can consider setting this option to
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slightly more than the expected completion time for an election. For example,
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:manual:`Replica Set Elections </core/replica-set-elections/>` states that
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elections will not typically exceed 12 seconds, so a 15-second timeout may be
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reasonable. Applications connecting to a sharded cluster may consider a smaller
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value still, since ``mongos`` insulates the driver from elections.
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That said, ``serverSelectionTimeoutMS`` should generally not be set to a value
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smaller than ``connectTimeoutMS``.
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serverSelectionTryOnce
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~~~~~~~~~~~~~~~~~~~~~~
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``serverSelectionTryOnce`` determines whether the driver should give up after
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the first failed server selection attempt or continue waiting until
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``serverSelectionTimeoutMS`` is reached. PHP defaults to ``true``, which allows
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the driver to "fail fast" when a server cannot be selected (e.g. no primary
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during a failover).
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The default behavior is generally desirable for a high-traffic web applications,
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as it means the worker process will not be blocked in a server selection loop
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and can instead return an error response and immediately go on to serve another
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request. Additionally, other driver features such as retryable reads and writes
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can still enable applications to avoid transient errors such as a failover.
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That said, applications that prioritize resiliency over response time (and
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worker pool utilization) may want to specify ``false`` for
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``serverSelectionTryOnce``.
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socketCheckIntervalMS
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~~~~~~~~~~~~~~~~~~~~~
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``socketCheckIntervalMS`` determines how often a socket should be checked (using
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a ``ping`` command) if it has not been used recently. This defaults to 5 seconds
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and is intentionally lower than ``heartbeatFrequencyMS`` to better allow
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single-threaded drivers to recover dropped connections.
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socketTimeoutMS
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~~~~~~~~~~~~~~~
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``socketTimeoutMS`` determines the maximum amount of time to spend reading or
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writing to a socket. Since server monitoring uses ``connectTimeoutMS`` for its
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socket timeouts, ``socketTimeoutMS`` only applies to operations executed by the
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application.
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``socketTimeoutMS`` defaults to 5 minutes; however, it's likely that a PHP web
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request would be terminated sooner due to
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`max_execution_time <https://www.php.net/manual/en/info.configuration.php#ini.max-execution-time>`_,
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which defaults to 30 seconds for web SAPIs. In a CLI environment, where
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``max_execution_time`` is unlimited by default, it is more likely that
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``socketTimeoutMS`` could be reached.
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.. note::
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``socketTimeoutMS`` is not directly related to server selection and
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monitoring; however, it is frequently associated with the other options and
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therefore bears mentioning.

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