Data durability and backup

Memgraph uses two mechanisms to ensure the durability of stored data and make disaster recovery possible:

write-ahead logging (WAL)
periodic snapshot creation

These mechanisms generate durability files and save them in the respective wal and snapshots folders in the data directory. Data directory stores permanent data on-disk.

The default data directory path is /var/lib/memgraph but the path can be changed by modifying the data-dir configuration flag.

If you are using a multi-tenant architecture, the data_directory also houses databases within /data_directory/databases/*db_name*.

Durability files are deleted when certain events are triggered, for example, exceeding the maximum number of snapshots, defined by the --storage-snapshot-retention-count=3 flag.

To prevent the deletion of durability files, you need to lock the data directory, and enable it again by unlocking the directory.

To manage this behavior, use the following queries:

LOCK DATA DIRECTORY;
UNLOCK DATA DIRECTORY;

To show the status of the data directory, run:

DATA DIRECTORY LOCK STATUS;

To encrypt the data directory, use LUKS (opens in a new tab) as it works with Memgraph out of the box and is undetectable from the applications perspective so it shouldn't break any existing applications.

Durability mechanisms

To configure the durability mechanisms check their respective configuration flags in the configuration settings.

Write-ahead logging

Write-ahead logging (WAL) is a technique applied in providing atomicity and durability to database systems.

In the default IN_MEMORY_TRANSACTIONAL storage mode, Memgraph creates a Delta object each time data is changed. By using Deltas, Memgraph creates write-ahead logs. Each database modification is therefore recorded in a log file before being written to the DB, and in the end the log file contains all steps needed to reconstruct the DB’s most recent state.

Memgraph has WAL enabled by default. To switch it on and off, use the boolean --storage-wal-enabled flag. For other WAL-related flags check the configuration reference guide.

WAL files are usually located at /var/lib/memgraph/wal.

Snapshots

Snapshots provide a faster way to restore the states of your database. Snapshots are created periodically based on the value defined with the --storage-snapshot-interval-sec configuration flag, as well as upon exit based on the value of the --storage-snapshot-on-exit configuration flag. When a snapshot creation is triggered, the entire data storage is written to the drive. Nodes and relationships are divided into groups called batches.

On startup, the database state is recovered from the most recent snapshot file. Memgraph can read the data and build the indexes on multiple threads, using batches as a parallelization unit: each thread will recover one batch at a time until there are no unhandled batches.

This means the same batch size might not be suitable for every dataset. A smaller dataset might require a smaller batch size to utilize a multi-threaded processor, while bigger datasets might use bigger batches to minimize the synchronization between the worker threads. Therefore the size of batches and the number of used threads are configurable similarly to other durability related settings.

The timestamp of the snapshot is compared with the latest update recorded in the WAL file and, if the snapshot is less recent, the state of the DB will be recovered using the WAL file.

Memgraph has snapshot creation enabled by default. You can configure the exact snapshot creation behavior by defining the relevant flags. Alternatively, you can make one directly by running the following query:

CREATE SNAPSHOT;

Snapshot files are saved inside the snapshots directory located in the data directory (var/lib/memgraph).

⚠️

Snapshots and WAL files are presently not compatible between Memgraph versions.

Create backup

Follow these steps to create database backup:

Create a snapshot

If necessary, create a snapshot of the current database state by running the following query in mgconsole or Memgraph Lab:

CREATE SNAPSHOT;

The snapshot is saved in the snapshots directory of the data directory (/var/lib/memgraph).

Lock the data directory

Durability files are deleted when an event is triggered, for example, exceeding the maximum number of snapshots.

To disable this behavior, run the following query in mgconsole or Memgraph Lab:

LOCK DATA DIRECTORY;

Copy files

Copy snapshot files (from the snapshots directory) and any additional WAL files (from the wal directory) to a backup location.

If you've just created a snapshot file there is no need to backup WAL files.

To copy the snapshot files from the Docker container first check the container ID by running docker ps then run the following command:

 docker cp  <CONTAINER ID>:/var/lib/memgraph/<snapshot_file> <snapshot_file>

Unlock the data directory

Run the following query in mgconsole or Memgraph Lab to unlock the directory:

UNLOCK DATA DIRECTORY;

Memgraph will delete the files which should have been deleted before locking and allow any future deletion of the durability files.

Restore data

Follow these steps to restore data from a backup:

Empty the `wal` directory

If you want to restore data only from the snapshot file, ensure that the wal directory is empty:

Find the container ID using a docker ps command, then enter the container using:

docker exec -it CONTAINER_ID bash

Position yourself in the /var/lib/memgraph/wal directory and rm *

Stop the instance

Run the following command

docker stop CONTAINER_ID

Start the instance

You can start the instance with the backed up files in two ways.

Option 1

You can start the instance by adding a -v ~/snapshots:/var/lib/memgraph/snapshots flag to the docker run command, where the ~/snapshots represents a path to the location of the directory with the back-up snapshot, for example:

docker run -p 7687:7687 -p 7444:7444 -v ~/snapshots:/var/lib/memgraph/snapshots memgraph/memgraph

If you want to copy both WAL and snapshot files start the instance by adding a -v ~/snapshots:/var/lib/memgraph/snapshots -v ~/wal:/var/lib/memgraph/wal flags to the docker run command, where the ~/snapshots represents a path to the location of the backed-up snapshot directory, and ~/wal represents a path to the location of the backed-up wal directory for example:

docker run -p 7687:7687 -p 7444:7444 -v ~/snapshots:/var/lib/memgraph/snapshots -v ~/wal:/var/lib/memgraph/wal memgraph/memgraph

Option 2

The other option is to copy the backed-up snapshot file into the snapshots directory after creating the container and start the database. So the commands should look like this:

docker create -p 7687:7687 -p 7444:7444 -v `snapshots`:/var/lib/memgraph/snapshots --name memgraphDB memgraph/memgraph
tar -cf - sample_snapshot_file | docker cp -a - memgraphDB:/var/lib/memgraph/snapshots

The sample_snapshot_file is the snapshot file you want to use to restore the data. Due to the nature of Docker file ownership, you need to use tar to copy the file as STDIN into the non-running container. It will allow you to change the ownership of the file to the memgraph user inside the container.

After that, start the database with:

docker start -a memgraphDB

The -a flag is used to attach to the container's output so you can see the logs.

Once memgraph is started, change the snapshot directory ownership to the memgraph user by running the following command:

docker exec -it -u 0 memgraphDB bash -c "chown memgraph:memgraph /var/lib/memgraph/snasphots"

Otherwise, Memgraph will not be able to write the future snapshot files and will fail.

Database dump

The database dump contains a record of the database state in the form of Cypher queries. It’s equivalent to the SQL dump in relational DBs.

You can run the queries constituting the dump to recreate the state of the DB as it was at the time of the dump.

To dump the Memgraph DB, run the following query:

DUMP DATABASE;

If you are using Memgraph Lab, you can dump the database, that is, the queries to recreate it, to a CYPHERL file in the Import & Export section of the Lab.

Storage modes

Memgraph has the option to work in IN_MEMORY_ANALYTICAL, IN_MEMORY_TRANSACTIONAL or ON_DISK_TRANSACTIONAL storage modes.

Memgraph always starts in the IN_MEMORY_TRANSACTIONAL mode in which it creates periodic snapshots and write-ahead logging as durability mechanisms, and also enables creating manual snapshots.

In the IN_MEMORY_ANALYTICAL mode, Memgraph offers no periodic snapshots and write-ahead logging. Users can create a snapshot with the CREATE SNAPSHOT; Cypher query. During the process of snapshot creation, other transactions will be prevented from starting until the snapshot creation is completed.

In the ON_DISK_TRANSACTIONAL mode, durability is supported by RocksDB since it keeps its own WAL (opens in a new tab) files. Memgraph persists the metadata used in the implementation of the on-disk storage.