Setting Up Master-Slave Replication Manually

To set up master-slave replication manually, follow this procedure.

  1. Ensure that you've followed these prerequisite steps:
    • Configure one or more gateways in the destination cluster. See Configuring and Managing MapR Gateways.
    • If the source and destination clusters are both secured, set up security for replication between the clusters. See Configuring MapR Clusters for Replication Between Tables.
    • Run the maprcli table info command on the source table to verify that you have the following permissions:
      • readperm, which is required for reading from the table.
      • replperm, which is required for replicating from the table.
    • Run the maprcli table info command on the destination table (if it already exists) to verify that you have the following permissions:
      • bulkload, which is required for the initial copy of source data into the destination table.
      • replperm, which is required for receiving replicated updates from the source table.
  2. Create the replica manually with the maprcli command table create. Use the -copyMetaFrom option to ensure that the metadata for the replica is identical to the metadata for the source table. Metadata includes column families, access control expressions (ACEs), and other attributes.
    maprcli table create -path <path to the replica> -copyMetaFrom <path to the source table>
    For example, to create the replica customers in the newyork cluster and use the metadata from the source table in the sanfrancisco cluster, you could use this command:
    maprcli table create -path /mapr/newyork/customers -copymetafrom /mapr/sanfrancisco/customers
  3. Register the replica as a replica of the source table by running the maprcli table replica add command.
    maprcli table replica add -path <path to the source table> -replica <path to the replica> -paused true
    For example, to register the customers table in the newyork cluster as a replica of the customers table in the sanfrancisco cluster, you could use this command:
    maprcli table replica add -path /mapr/sanfrancisco/customers -replica /mapr/newyork/customers -paused true

    The -paused parameter ensures that replication does not start immediately after you register the source table as a source for this replica. You do this registration in step 4.

    You can use these optional parameters in this command:
    -columns
    For binary tables
    Provide a comma-separated list of column families or columns from a certain column family (column family:qualifier). For example, use the following syntax to replicate the column family purchases and the column stars in the reviews column family: -columns purchases,reviews:stars
    Note: While the column families that you specify must already exist in the source table, the columns that you specify do not have to exist in the destination table for replication to succeed. If the column is added at a later date, replication for that column will start at that time.
    For JSON tables
    Provide a comma-delimited list of fields to replicate. Include the full field path for each field.

    Example

    Suppose your table contains documents that contain this general structure:

    {
         "_id" : "ID",
         "a" :
              {
                   "b" : 
                        {
                             "c" : "value",
                        },
                   "e" : "value"
              }
    }

    To replicate fields a, c, and e, you would specify these field paths:

    a,a.b.c,a.e
    Do not use quotation marks and do not include spaces after commas.

    Suppose now that a.b and a.e were custom column families. You want to replicate only the default column family and column family a.e. To do so, you would specify field paths like this:

    ",a.e"

    The empty string before the comma indicates the default column family.

    -synchronous
    This parameter specifies whether replication is synchronous or asynchronous. Asynchronous is the default. The values are yes for synchronous and no for asynchronous.
  4. Verify that you specified the correct replica by running the maprcli table replica list command.
    maprcli table replica list -path <path to the source table>
    To verify that the customers table in the newyork cluster is a replica of the customers table in the sanfrancisco cluster, you could look at the output of this command:
    maprcli table replica list -path /mapr/sanfrancisco/customers 
  5. Authorize replication between the tables by defining the source table as the upstream table for the replica by running the maprcli table upstream add command. Definition of the upstream table ensures that a table cannot replicate updates to any replica. Replication depends on a two-way agreement between the owners of the two tables.
    maprcli table upstream add -path <path to the replica> -upstream <path to the source table>
    To add the customers table in the sanfrancisco cluster as an upstream source for the customers table in the newyork cluster:
    maprcli table upstream add -path /mapr/newyork/customers -upstream /mapr/sanfrancisco/customers
  6. Verify that you specified the correct source table by running the maprcli table upstream list command.
    maprcli table upstream list -path <path to the replica>
    To verify this in our example scenario, you could use this command:
    maprcli table upstream list -path /mapr/newyork/customers
  7. If you set -paused to true when adding the replica, follow these steps:
    1. Load the replica with data from the source table by using the MapR-DB CopyTable utility for binary tables or the mapr copytable utility for JSON tables.
    2. Start replication with the command maprcli table replica resume. Here is the maprcli command:
      maprcli table replica resume -path <path to the source table> -replica <path to the replica>
      For our example scenario, you could use this command:
      maprcli table replica resume -path mapr/sanfrancisco/customers -replica /mapr/newyork/customers
  • Be aware that changes to the structure of a source table are not replicated automatically to replicas. For example, if a new column family is added to the source table and the entire table is being replicated (i.e. the maprcli table replica add command did not specify column families or columns to replicate), the new column family is not automatically created at the replica.

    You can add the new column family to the replica only if the entire source table is being replicated, then updates to the new column family will immediately start being replicated. You do not need to carry out the next steps. Continue only if you are replicating a subset of column families and columns.

    If you are replicating a subset of column families and columns, follow these steps to add a new column family to the replica:
    1. Pause replication by running the maprcli table replica pause command.
    2. Add the new column family to the replica by running the maprcli table replica edit command.
    3. Copy the data for this column family from the source table into the replica by using the MapR-DB CopyTable utility. Use the -columns parameter to specify the name of the column family.
    4. Resume replication by running the maprcli table replica resume command.
  • Check for alarms related to replication and whether synchronous replication is switched temporarily to asynchronous replication by looking in MCS. See Table-Replication Alarms.