Recovering From MySQL Cluster Downtime

When to Bootstrap
Run the Bootstrap Errand

Determine Type of Cluster Failure
Scenario 1: Virtual Machines Running, Cluster Disrupted
Scenario 2: Virtual Machines Terminated or Lost

Bootstrap Manually

Shut Down MySQL
Choose Node to Bootstrap
Bootstrap the First Node
Restart Remaining Nodes

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This topic describes how to bootstrap your MySQL cluster in the event of a cluster failure.

You can bootstrap your cluster by using one of two methods:

Run the bootstrap errand. See Run the Bootstrap Errand below.
Bootstrap manually. See Bootstrap Manually below.

When to Bootstrap

You must bootstrap a cluster that loses quorum. A cluster loses quorum when less than half of the nodes can communicate with each other for longer than the configured grace period. If a cluster does not lose quorum, individual unhealthy nodes automatically rejoin the cluster after resolving the error, restarting the node, or restoring connectivity.

To check whether your cluster has lost quorum, look for the following symptoms:

All nodes appear “Unhealthy” on the proxy dashboard, viewable at https://BOSH-JOB-INDEX-proxy-p-mysql-ert.YOUR-SYSTEM-DOMAIN:

All responsive nodes report the value of wsrep_cluster_status as non-Primary:

mysql> SHOW STATUS LIKE 'wsrep_cluster_status';
+----------------------+-------------+
| Variable_name        | Value       |
+----------------------+-------------+
| wsrep_cluster_status | non-Primary |
+----------------------+-------------+

All unresponsive nodes respond with ERROR 1047 when using most statement types in the MySQL client:

mysql> select * from mysql.user;
ERROR 1047 (08S01) at line 1: WSREP has not yet prepared node for application use

For more information about checking the state of your cluster, see Check Cluster State.

Run the Bootstrap Errand

MySQL for PCF includes a BOSH errand that automates the manual bootstrapping procedure in the Bootstrap Manually section below.

It finds the node with the highest transaction sequence number and asks it to start up by itself (i.e. in bootstrap mode) and then asks the remaining nodes to join the cluster.

In most cases, running the errand recovers your cluster. But, certain scenarios require additional steps.

Determine Type of Cluster Failure

To determine which set of instructions to follow, do the following:

Run the following command.
```
bosh -e YOUR-ENV -d YOUR-DEPLOYMENT instances
```
Where:
- YOUR-ENV is the environment where you deployed the cluster.
- YOUR-DEPLOYMENT is the deployment cluster name.
For example:
```
$ bosh -e prod -d mysql instances
```

Find and record the Process State for your MySQL instances. In the following example output, the MySQL instances are in the failing process state.

Instance                                                             Process State  AZ             IPs
backup-restore/c635410e-917d-46aa-b054-86d222b6d1c0                  running        us-central1-b  10.0.4.47
bootstrap/a31af4ff-e1df-4ff1-a781-abc3c6320ed4                       -              us-central1-b  -
broker-registrar/1a93e53d-af7c-4308-85d4-3b2b80d504e4                -              us-central1-b  10.0.4.58
cf-mysql-broker/137d52b8-a1b0-41f3-847f-c44f51f87728                 running        us-central1-c  10.0.4.57
cf-mysql-broker/28b463b1-cc12-42bf-b34b-82ca7c417c41                 running        us-central1-b  10.0.4.56
deregister-and-purge-instances/4cb93432-4d90-4f1d-8152-d0c238fa5aab  -              us-central1-b  -
monitoring/f7117dcb-1c22-495e-a99e-cf2add90dea9                      running        us-central1-b  10.0.4.48
mysql/220fe72a-9026-4e2e-9fe3-1f5c0b6bf09b                           failing        us-central1-b  10.0.4.44
mysql/28a210ac-cb98-4ab4-9672-9f4c661c57b8                           failing        us-central1-f  10.0.4.46
mysql/c1639373-26a2-44ce-85db-c9fe5a42964b                           failing        us-central1-c  10.0.4.45
proxy/87c5683d-12f5-426c-b925-62521529f64a                           running        us-central1-b  10.0.4.60
proxy/b0115ccd-7973-42d3-b6de-edb5ae53c63e                           running        us-central1-c  10.0.4.61
rejoin-unsafe/8ce9370a-e86b-4638-bf76-e103f858413f                   -              us-central1-b  -
smoke-tests/e026aaef-efd9-4644-8d14-0811cb1ba733                     -              us-central1-b  10.0.4.59

Choose your scenario:
- If your MySQL instances are in the failing state, continue to Scenario 1.
- If your MySQL instances are in the - state, continue to Scenario 2.

Scenario 1: VMs Running, Cluster Disrupted

In this scenario, the VMs are running, but the cluster has been disrupted.

To bootstrap in this scenario, follow these steps:

To run the bootstrap errand, run the following command:

bosh -e YOUR-ENV -d YOUR-DEPLOYMENT run-errand bootstrap

Where:

YOUR-ENV is the name of your environment.
YOUR-DEPLOYMENT is the name of your deployment.

Note: The errand runs for a long time, during which no output is returned.

The command returns many lines of output, eventually followed by:

Bootstrap errand completed
[stderr]
+ echo 'Started bootstrap errand ...'
+ JOB_DIR=/var/vcap/jobs/bootstrap
+ CONFIG_PATH=/var/vcap/jobs/bootstrap/config/config.yml
+ /var/vcap/packages/bootstrap/bin/cf-mysql-bootstrap -configPath=/var/vcap/jobs/bootstrap/config/config.yml
+ echo 'Bootstrap errand completed'
+ exit 0
Errand 'bootstrap' completed successfully (exit code 0)

If the errand fails, run the bootstrap errand command again after a few minutes. The bootstrap errand might not work the first time.
If the errand fails after several tries, bootstrap your cluster manually. See Bootstrap Manually below.

Scenario 2: VMs Terminated or Lost

In severe circumstances, such as a power failure, it is possible to lose all your VMs. You must recreate them before you can begin recovering the cluster.

When MySQL instances are in the - state, the VMs are lost. The procedures in this scenario bring the instances from a - state to a failing state. Then you run the bootstrap errand similar to Scenario 1 above and restore configuration.

To recover terminated or lost VMs, do the procedures in the sections below:

Recreate the Missing VMs: Bring MySQL instances from a - state to a failing state.
Run the Bootstrap Errand: Since your instances are now in the failing state, you continue similarly to Scenario 1 above.
Restore the BOSH Configuration: Go back to unignoring all instances and redeploy. This is a critical and mandatory step.

WARNING: If you do not set each of your ignored instances to unignore, your instances are not updated in future deploys. You must perform the procedure in the final section of Scenario 2, Restore the BOSH Configuration.

Recreate the Missing VMs

The procedure in this section uses BOSH to recreate the VMs, install software on them, and try to start the jobs.

The procedure below allows you to do the following:

Redeploy your cluster while expecting the jobs to fail.
Instruct BOSH to ignore the state of each instance in your cluster. This allows BOSH to deploy the software to each instance even if the instance is failing.

To recreate your missing VMs, do the following:

SSH into the BOSH VMs using the procedure in BOSH SSH.
If BOSH resurrection is enabled, disable it by running the following command:
```
bosh -e YOUR-ENV update-resurrection off
```
Where YOUR-ENV is the name of your environment.
To download the current manifest, run the following command:
```
bosh -e YOUR-ENV -d YOUR-DEPLOYMENT manifest > /tmp/manifest.yml
```
Where:
- YOUR-ENV is the name of your environment.
- YOUR-DEPLOYMENT is the name of your deployment.
To redeploy deployment, run the following command:
```
bosh -e YOUR-ENV -d YOUR-DEPLOYMENT deploy /tmp/manifest.yml
```
Where:
- YOUR-ENV is the name of your environment.
- YOUR-DEPLOYMENT is the name of your deployment.
Note: Expect one of the MySQL VMs to fail. Deploying causes BOSH to create new VMs and install the software. Forming a cluster is in a subsequent step.
To view the instance GUID of the VM that attempted to start, run the following command:
```
  bosh -e YOUR-ENV -d YOUR-DEPLOYMENT instances
```
Where:
- YOUR-ENV is the name of your environment.
- YOUR-DEPLOYMENT is the name of your deployment.
Record the instance GUID, which is the string after mysql/ in your BOSH instances output.
To instruct BOSH to ignore each MySQL VM, run the following command:
```
bosh -e YOUR-ENV -d YOUR-DEPLOYMENT ignore mysql/INSTANCE-GUID
```
Where:
- YOUR-ENV is the name of your environment.
- YOUR-DEPLOYMENT is the name of your deployment.
- INSTANCE-GUID is the GUID of your instance you recorded in the above step.
Repeat steps 4 through 6 until all instances have attempted to start.
If you disabled BOSH resurrection in step 2, to re-enable it, run the following command:
```
bosh -e YOUR-ENV update-resurrection on
```
Where YOUR-ENV is the name of your environment.
To confirm that your MySQL instances have gone from the - state to the failing state, run the following command
```
bosh -e YOUR-ENV -d YOUR-DEPLOYMENT instances
```
Where:
- YOUR-ENV is the name of your environment.
- YOUR-DEPLOYMENT is the name of your deployment.

Run the Bootstrap Errand

After you recreate the VMs, all instances now have a failing process state and have the MySQL code. You must run the bootstrap errand to recover the cluster.

To bootstrap, do the following:

To run the bootstrap errand, run the following command:

bosh -e YOUR-ENV -d YOUR-DEPLOYMENT run-errand bootstrap

Where:

YOUR-ENV is the name of your environment.
YOUR-DEPLOYMENT is the name of your deployment.

Note: The errand runs for a long time, during which no output is returned.

The command returns many lines of output, eventually with the following successful output:

  Bootstrap errand completed
  [stderr]
  echo 'Started bootstrap errand ...'
  JOB_DIR=/var/vcap/jobs/bootstrap
  CONFIG_PATH=/var/vcap/jobs/bootstrap/config/config.yml
  /var/vcap/packages/bootstrap/bin/cf-mysql-bootstrap -configPath=/var/vcap/jobs/bootstrap/config/config.yml
  echo 'Bootstrap errand completed'
  exit 0
  Errand 'bootstrap' completed successfully (exit code 0)

If the errand fails, run the bootstrap errand command again after a few minutes. The bootstrap errand might not work immediately.
See that the errand completes successfully in the shell output and continue to Restore the BOSH Configuration below.
Note: After you complete the bootstrap errand, you might still see instances in the failing state. Continue to the next section anyway.

Restore the BOSH Configuration

WARNING: If you do not set each of your ignored instances to unignore, your instances are never updated in future deploys.

To restore your BOSH configuration to its previous state, this procedure unignores each instance that was previously ignored:

For each ignored instance, run the following command:
```
bosh -e YOUR-ENV -d YOUR-DEPLOYMENT unignore mysql/INSTANCE-GUID
```
Where:
- YOUR-ENV is the name of your environment.
- YOUR-DEPLOYMENT is the name of your deployment.
- INSTANCE-GUID is the GUID of your instance.
To redeploy your deployment, run the following command:
```
bosh -e YOUR-ENV -d YOUR-DEPLOYMENT deploy
```
Where:
- YOUR-ENV is the name of your environment.
- YOUR-DEPLOYMENT is the name of your deployment.
To validate that all mysql instances are in a running state, run the following command:
```
bosh -e YOUR-ENV -d YOUR-DEPLOYMENT instances
```
Where:
- YOUR-ENV is the name of your environment.
- YOUR-DEPLOYMENT is the name of your deployment.

Bootstrap Manually

If the bootstrap errand is not able to automatically recover the cluster, you might need to do the steps manually.

Do the procedures in the sections below to manually bootstrap your cluster.

WARNING: The following procedures are prone to user-error and can result in lost data if followed incorrectly. Follow the procedure in Bootstrap with the BOSH Errand above first, and only resort to the manual process if the errand fails to repair the cluster.

Shut Down MySQL

Do the following for each node in the cluster:

SSH into the node using the procedure in BOSH SSH.
To shut down the mysqld process on the node, run the following command:
```
monit stop galera-init
```

Re-bootstrapping the cluster is not successful unless you shut down the mysqld process on all nodes in the cluster.

Choose Node to Bootstrap

To avoid losing data, you must bootstrap from a node in the cluster that has the highest transaction sequence number (seqno).

For each node in the cluster, do the following to find its seqno:

SSH into the node using the procedure in BOSH SSH.
Run the following command to find seqno values in the node’s Galera state file, grastate.dat: cat /var/vcap/store/pxc-mysql/grastate.dat | grep 'seqno:'
Do one of the following, based on the last and highest seqno value in the grep output above:
- If seqno is a positive integer, then the node shut down gracefully. Record this number for comparison with the latest seqno of other nodes in the cluster.
- If seqno is -1, then node crashed or was killed. Proceed as follows to possibly recover the seqno from the database:
  1. Run the following command to temporarily start the database and append the last sequence number to its error log: /var/vcap/packages/pxc/bin/mysqld --defaults-file=/var/vcap/jobs/pxc-mysql/config/my.cnf --wsrep-recover
  2. Scan the end of the error log mysql.err.log for the recovered seqno. It appears after the group ID (a UUID) and a colon. For example, it is 15026 in the following output:
```
$ grep "Recovered position" /var/vcap/sys/log/pxc-mysql/mysql.err.log | tail -1
150225 18:09:42 mysqld_safe WSREP: Recovered position e93955c7-b797-11e4-9faa-9a6f0b73eb46:15026
```
  3. If the node never connected to the cluster before crashing, it may never have been assigned a group ID. In this case, there is nothing to recover. Do not choose this node for bootstrapping unless all the other nodes also nodes crashed.
After you determine the seqno for all nodes in your cluster, identify the one with the highest seqno. If multiple nodes share the same highest seqno, and it is not -1, you can bootstrap from any of them.

Bootstrap the First Node

After determining the node with the highest seqno, do the following to bootstrap the node:

Note: Only run these bootstrap commands on the node with the highest seqno. Otherwise the node with the highest seqno is unable to join the new cluster unless its data is abandoned. Its mysqld process exits with an error. For more information about intentionally abandoning data, see Architecture.

SSH into the node using the procedure in BOSH SSH.
To update the state file and restart the mysqld process on the new bootstrap node, run the following commands:
```
echo -n "NEEDS_BOOTSTRAP" > /var/vcap/store/pxc-mysql/state.txt
monit start galera-init
```
It can take up to ten minutes for monit to start the mysqld process. To check if the mysqld process has started successfully, run the following command:
```
watch monit summary
```

Restart Remaining Nodes

After the bootstrapped node is running, to restart the nodes do the following:

SSH into the node using the procedure in BOSH SSH.
To start the mysqld process with monit, run the following command:
```
monit start galera-init
```
If the node is prevented from starting by the Interruptor, do the manual procedure to force the node to rejoin the cluster, documented in How to Manually Force a MySQL Node to Rejoin if a Node Cannot Rejoin the Cluster for P-MySQL 1.9 and 1.10 in the Pivotal Support knowledge base.

WARNING: Forcing a node to rejoin the cluster is a destructive procedure. Only do the procedure with the assistance of Pivotal Support.
If the monit start command fails, it might be because the node with the highest seqno is mysql/0.

In this case, do the following:
1. To make BOSH ignore updating mysql/0, run the following command:
```
bosh -e YOUR-ENV -d YOUR-DEPLOYMENT ignore mysql/0
```
  Where:
  - YOUR-ENV is the name of your environment.
  - YOUR-DEPLOYMENT is the name of your deployment.
2. Navigate to Ops Manager in a browser, log in, and click Apply Changes.
3. When the deploy finishes, run the following command from the Ops Manager VM:
```
bosh -e YOUR-ENV -d YOUR-DEPLOYMENT unignore mysql/0
```
  Where:
  - YOUR-ENV is the name of your environment.
  - YOUR-DEPLOYMENT is the name of your deployment.
To verify that the new nodes have successfully joined the cluster, SSH into the bootstrap node and run the following command to output the total number of nodes in the cluster:
```
mysql> SHOW STATUS LIKE 'wsrep_cluster_size';
```

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