Mongo Locks Test

MongoDB allows multiple clients to read and write the same data. In order to ensure consist ency, it uses locking to prevent multiple clients from modifying the same piece of data simultaneously.

MongoDB uses reader-writer locks that allow concurrent readers shared access to a resource, such as a database or collection, but in MMAPv1, give exclusive access to a single write operation.

In addition to a shared (S) locking mode for reads and an exclusive (X) locking mode for write operations, intent shared (IS) and intent exclusive (IX) modes indicate an intent to read or write a resource using a finer granularity lock. When locking at a certain granularity, all higher levels are locked using an intent lock.

For example, when locking a collection for writing (using mode X), both the corresponding database lock and the global lock must be locked in intent exclusive (IX) mode. A single database can simultaneously be locked in IS and IX mode, but an exclusive (X) lock cannot coexist with any other modes, and a shared (S) lock can only coexist with intent shared (IS) locks.

Locks are fair, with reads and writes being queued in order. However, to optimize throughput, when one request is granted, all other compatible requests will be granted at the same time, potentially releasing them before a conflicting request. For example, consider a case in which an X lock was just released, and in which the conflict queue contains the following items:

IS → IS → X → X → S → IS

In strict first-in, first-out (FIFO) ordering, only the first two IS modes would be granted. Instead MongoDB will actually grant all IS and S modes, and once they all drain, it will grant X, even if new IS or S requests have been queued in the meantime.

Sometimes, in an attempt to release locks for compatible requests, MongoDB may end up increasing the lock wait times for certain incompatible requests. Some other times, certain long-running operations can cause the length of the queue to increase along with the waiting time for locks. Long lock wait times can adversely impact application performance. This is why, administrators will have to keep an eye on the locking activity on a MongoDB server, determine whether/not requests are waiting too long for locks, and also identify the types of locks (S, X, IS, IX, etc.) these requests are waiting for. This will enable administrators to quickly diagnose why certain lock types are taking longer to be released, and resolve the bottleneck. The Mongo Locks test helps administrators achieve this.

This test auto-discovers the different lock types/lock modes that exist currently on the target server. For each lock type/mode, the test reports the rate at which locks of that type were acquired, the rate at which requests waited for locks of that type, and the average wait time for the locks. In the process, the test promptly alerts administrators to prolonged waiting time for locks and also pinpoints the exact lock types for which requests are waiting too long. With this information, administrators can easily troubleshoot long wait times.

Target of the test : A MongoDB server

Agent deploying the test : An internal/remote agent

Outputs of the test : One set of results for each lock type / lock mode.

Configurable parameters for the test

Parameter	Description
Test period	How often should the test be executed.
Host	The host for which the test is to be configured.
Port	The port number at which the specified host listens.
Database Name	The test connects to a specific Mongo database to run API commands and pull metrics of interest. Specify the name of this database here. The default value of this parameter is admin.
Username and Password	The eG agent has to be configured with the credentials of a user who has the required privileges to monitor the target MongoDB instance, if the MongoDB instance is access control enabled. To know how to create such a user, refer to How to monitor access control enabled MongoDB database?. If the target MongoDB instance is not access control enabled, then, specify none against the Username and Password parameters.
Confirm Password	Confirm the password by retyping it here.
Authentication Mechanism	Typically, the MongoDB supports multiple authentication mechanisms that users can use to verify their identity. In environments where multiple authentication mechanisms are used, this test enables the users to select the authentication mechanism of their interest using this list box. By default, this is set to None. However, you can modify this settings as per the requirement.
SSL	By default, the SSL flag is set to No, indicating that the target MongoDB server is not SSL-enabled by default. To enable the test to connect to an SSL-enabled MongoDB server, set the SSL flag to Yes.
CA File	A certificate authority (CA) file contains root and intermediate certificates that are electronically signed to affirm that a public key belongs to the owner named in the certificate. If you are looking to monitor the certificates contained within a CA file, then provide the full path to this file in the CA File text box. For example, the location of this file may be: C:\cert\rootCA.pem. If you do not want to monitor the certificates in a CA file, set this parameter to none.
Certificate Key File	A Certificate Key File specifies the path on the server where your private key is stored. If you are looking to monitor the Certificate Key File, then provide the full path to this file in the Certificate Key File text box. For example, the location of this file may be: C:\cert\mongodb.pem. If you do not want to monitor the certificates in a CA file, set this parameter to none.

Measurements made by the test

Measurement	Description	Measurement Unit	Interpretation
Locks	Indicates the rate at which a lock of this type was acquired.	Locks/Sec
Lock waits	Indicates the rate at which requests were waiting to acquire a lock of this type.	Waits/Sec	A consistent increase in the value for this measure could indicate that locks are not being released in a timely manner.
Lock wait time	Indicates the time period for which requests were waiting to acquire a lock of this type, during the last measurement period.	Seconds
Avg lock wait time	Indicates the average time for which requests were waiting to acquire a lock of this type.	Seconds	A high average lock wait time may mean sessions are having to wait for a long time to acquire locks on objects. A high value may indicate one of the following: Too many transactions happening Locked resources not being released properly Locks are being held by long-running operations