Troubleshoot Replica Sets

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Check Replica Set Status

Check the Replication Lag
Slow Application of Oplog Entries
Test Connections Between all Members
Socket Exceptions when Rebooting More than One Secondary
Check the Size of the Oplog

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This section describes common strategies for troubleshooting replica set deployments.

Check Replica Set Status

To display the current state of the replica set and current state of each member, run the rs.status() method in a mongo shell connected to the replica set's primary. For descriptions of the information displayed by rs.status(), see replSetGetStatus.

Note

The rs.status() method is a wrapper that runs the replSetGetStatus database command.

Check the Replication Lag

Replication lag is a delay between an operation on the primary and the application of that operation from the oplog to the secondary. Replication lag can be a significant issue and can seriously affect MongoDB replica set deployments. Excessive replication lag makes "lagged" members ineligible to quickly become primary and increases the possibility that distributed read operations will be inconsistent.

To check the current length of replication lag:

In a mongo shell connected to the primary, call the rs.printSecondaryReplicationInfo() method.

Returns the syncedTo value for each member, which shows the time when the last oplog entry was written to the secondary, as shown in the following example:

source: m1.example.net:27017
    syncedTo: Thu Apr 10 2014 10:27:47 GMT-0400 (EDT)
    0 secs (0 hrs) behind the primary
source: m2.example.net:27017
    syncedTo: Thu Apr 10 2014 10:27:47 GMT-0400 (EDT)
    0 secs (0 hrs) behind the primary

A delayed member may show as 0 seconds behind the primary when the inactivity period on the primary is greater than the members[n].slaveDelay value.

Note

The rs.status() method is a wrapper around the replSetGetStatus database command.

Monitor the rate of replication by checking for non-zero or increasing oplog time values in the Replication Lag graph available in Cloud Manager and in Ops Manager.

Replication Lag Causes

Possible causes of replication lag include:

Network Latency
Check the network routes between the members of your set to ensure that there is no packet loss or network routing issue.
Use tools including ping to test latency between set members and traceroute to expose the routing of packets network endpoints.
Disk Throughput
If the file system and disk device on the secondary is unable to flush data to disk as quickly as the primary, then the secondary will have difficulty keeping state. Disk-related issues are incredibly prevalent on multi-tenant systems, including virtualized instances, and can be transient if the system accesses disk devices over an IP network (as is the case with Amazon's EBS system.)
Use system-level tools to assess disk status, including iostat or vmstat.
Concurrency
In some cases, long-running operations on the primary can block replication on secondaries. For best results, configure write concern to require confirmation of replication to secondaries. This prevents write operations from returning if replication cannot keep up with the write load.
You can also use the database profiler to see if there are slow queries or long-running operations that correspond to the incidences of lag.
Appropriate Write Concern
If you are performing a large data ingestion or bulk load operation that requires a large number of writes to the primary, particularly with unacknowledged write concern, the secondaries will not be able to read the oplog fast enough to keep up with changes.
To prevent this, request write acknowledgement write concern after every 100, 1,000, or another interval to provide an opportunity for secondaries to catch up with the primary.
For more information see:

Flow Control

Starting in MongoDB 4.2, administrators can limit the rate at which the primary applies its writes with the goal of keeping the majority committed lag under a configurable maximum value flowControlTargetLagSeconds.

By default, flow control is enabled.

Note

For flow control to engage, the replica set/sharded cluster must have: featureCompatibilityVersion (fCV) of 4.2 and read concern majority enabled. That is, enabled flow control has no effect if fCV is not 4.2 or if read concern majority is disabled.

With flow control enabled, as the lag grows close to the flowControlTargetLagSeconds, writes on the primary must obtain tickets before taking locks to apply writes. By limiting the number of tickets issued per second, the flow control mechanism attempts to keep the the lag under the target.

For information on flow control statistics, see:

Slow Application of Oplog Entries

Starting in version 4.2, secondary members of a replica set now log oplog entries that take longer than the slow operation threshold to apply. These slow oplog messages:

Are logged for the secondaries in the diagnostic log.
Are logged under the REPL component with the text applied op: <oplog entry> took <num>ms.
Do not depend on the log levels (either at the system or component level)
Do not depend on the profiling level.
May be affected by slowOpSampleRate, depending on your MongoDB version:
- In MongoDB 4.2, these slow oplog entries are not affected by the slowOpSampleRate. MongoDB logs all slow oplog entries regardless of the sample rate.
- In MongoDB 4.4 and later, these slow oplog entries are affected by the slowOpSampleRate.

The profiler does not capture slow oplog entries.

Test Connections Between all Members

All members of a replica set must be able to connect to every other member of the set to support replication. Always verify connections in both "directions." Networking topologies and firewall configurations can prevent normal and required connectivity, which can block replication.

Starting in MongoDB 3.6, MongoDB binaries, mongod and mongos, bind to localhost by default. If the net.ipv6 configuration file setting or the --ipv6 command line option is set for the binary, the binary additionally binds to the localhost IPv6 address.

Previously, starting from MongoDB 2.6, only the binaries from the official MongoDB RPM (Red Hat, CentOS, Fedora Linux, and derivatives) and DEB (Debian, Ubuntu, and derivatives) packages bind to localhost by default.

When bound only to the localhost, these MongoDB 3.6 binaries can only accept connections from clients (including the mongo shell, other members in your deployment for replica sets and sharded clusters) that are running on the same machine. Remote clients cannot connect to the binaries bound only to localhost.

To override and bind to other ip addresses, you can use the net.bindIp configuration file setting or the --bind_ip command-line option to specify a list of hostnames or ip addresses.

Warning

Before binding to a non-localhost (e.g. publicly accessible) IP address, ensure you have secured your cluster from unauthorized access. For a complete list of security recommendations, see Security Checklist. At minimum, consider enabling authentication and hardening network infrastructure.

For example, the following mongod instance binds to both the localhost and the hostname My-Example-Associated-Hostname, which is associated with the ip address 198.51.100.1:

mongod --bind_ip localhost,My-Example-Associated-Hostname

In order to connect to this instance, remote clients must specify the hostname or its associated ip address 198.51.100.1:

mongo --host My-Example-Associated-Hostname
mongo --host 198.51.100.1

Consider the following example of a bidirectional test of networking:

Example

Given a replica set with three members running on three separate hosts:

m1.example.net
m2.example.net
m3.example.net

All three use the default port 27017.

Test the connection from m1.example.net to the other hosts with the following operation set m1.example.net:
```
mongo --host m2.example.net --port 27017
mongo --host m3.example.net --port 27017
```
Test the connection from m2.example.net to the other two hosts with the following operation set from m2.example.net, as in:
```
mongo --host m1.example.net --port 27017
mongo --host m3.example.net --port 27017
```
You have now tested the connection between m2.example.net and m1.example.net in both directions.
Test the connection from m3.example.net to the other two hosts with the following operation set from the m3.example.net host, as in:
```
mongo --host m1.example.net --port 27017
mongo --host m2.example.net --port 27017
```

If any connection, in any direction fails, check your networking and firewall configuration and reconfigure your environment to allow these connections.

Socket Exceptions when Rebooting More than One Secondary

When you reboot members of a replica set, ensure that the set is able to elect a primary during the maintenance. This means ensuring that a majority of the set's members[n].votes are available.

When a set's active members can no longer form a majority, the set's primary steps down and becomes a secondary. The primary does not close client connections when it steps down.

Clients cannot write to the replica set until the members elect a new primary.

Example

Given a three-member replica set where every member has one vote, the set can elect a primary if at least two members can connect to each other. If you reboot the two secondaries at once, the primary steps down and becomes a secondary. Until at least another secondary becomes available, i.e. at least one of the rebooted secondaries also becomes available, the set has no primary and cannot elect a new primary.

For more information on votes, see Replica Set Elections. For related information on connection errors, see Does TCP keepalive time affect MongoDB Deployments?.

Check the Size of the Oplog

A larger oplog can give a replica set a greater tolerance for lag, and make the set more resilient.

To check the size of the oplog for a given replica set member, connect to the member in a mongo shell and run the rs.printReplicationInfo() method.

The output displays the size of the oplog and the date ranges of the operations contained in the oplog. In the following example, the oplog is about 10 MB and is able to fit about 26 hours (94400 seconds) of operations:

configured oplog size:   10.10546875MB
log length start to end: 94400 (26.22hrs)
oplog first event time:  Mon Mar 19 2012 13:50:38 GMT-0400 (EDT)
oplog last event time:   Wed Oct 03 2012 14:59:10 GMT-0400 (EDT)
now:                     Wed Oct 03 2012 15:00:21 GMT-0400 (EDT)

The oplog should be long enough to hold all transactions for the longest downtime you expect on a secondary. [1] At a minimum, an oplog should be able to hold minimum 24 hours of operations; however, many users prefer to have 72 hours or even a week's work of operations.

For more information on how oplog size affects operations, see:

Note

You normally want the oplog to be the same size on all members. If you resize the oplog, resize it on all members.

To change oplog size, see the Change the Size of the Oplog tutorial.

[1]	The oplog can grow past its configured size limit to avoid deleting the `majority commit point`.

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