Different types of index structures for storing database data in a replica group

A method, comprising:

• receiving a request to perform an update to a database at a leader node of a replica group for at least a portion of the database, wherein the leader node is associated with a first type of index structure;
• responsive to receiving the request:
• performing, by the leader node, the update to a first copy of the portion of the database at the leader node of the replica group stored in the first type of index structure;
• adding, by the leader node, a description of the update to a log of updates for the database sent by the leader node to a follower node of the replica group, wherein the follower node is associated with a second type of index structure that is different from the first type of index structure;
• before a size of the log of updates received at the follower node exceeds a threshold, storing, by the follower node, the log of updates at a data store that is local to the follower node, wherein the log of updates is stored at the data store that is local to the follower node for subsequently receiving and storing one or more descriptions of additional updates until the size of the log of updates is no longer below the threshold;
• and
• after the size of the log of updates at the follower node exceeds the threshold, storing, by the follower node, a copy of the log of updates according to the second type of index structure at an external storage and as part of a second copy of the portion of the database and updating the second copy of the portion of the database according to the copy of the log of updates.

One or more non-transitory, computer-readable storage media, storing program instructions that when executed on or across one or more computing devices cause the one or more computing devices to implement:

• receiving, at a follower node of a replica group, one or more updates from a log of updates, wherein the one or more updates from the log of updates are for at least a portion of a database, wherein the one or more updates are generated by a leader node for the replica group, wherein the leader node stores a first copy of at least a portion of the database according to a first type of index structure, wherein the leader node is associated with the first type of index structure, and wherein the follower node is associated with a second type of index structure that is different from the first type of index structure;
• and
• responsive to a determination that the log of updates received at the follower node is below a threshold, storing, by the follower node, the log of updates at a data store that is local to the follower node, wherein the log of updates is stored at the data store that is local to the follower node for subsequently receiving and storing one or more additional updates until the size of the log of updates is no longer below the threshold;
• and
• responsive to determining that the log of updates at the follower node meets or exceeds the threshold, storing, by the follower node, a copy of the log of updates according to the second type of index structure at an external storage system and as part of a second copy of the portion of the database and updating the second copy of the portion of the database according to the copy of the log of updates.