Best practice to handle table migrations?

Question

What is the best practice for migrating table when a member of a struct is added? For some structs, I have had no problems adding members, but for others I have. I get this error message:

$cleos get table <account> <contract> <table>
2628662ms thread-0   main.cpp:2552                 main                 ] Failed with error: Out of Range (8)
read datastream of length 16 over by 1
error unpacking uint32_t

This occurred after I added a uint32_t member to the table struct. Is there a way to configure the migration to the new table so that I don't have start over? I would want to default the value to zero or some other value.

Answer 1

When making structural changes to tables there are 2 basic approaches:

1. add versioning in the data from day 1 and then support old versions of the data well enough to do "just in time" upgrades.

2. deploy a migration contract in-between version N and N+1 which can read the data and convert it, OR ingests transactions with data converted in an out-of-chain process; then issue the setcode with the N+1 code

Both of these require some pre-planning (like the ability to put your contract into a maintenance mode for user feedback)

Answer 2

Binary_extension is a good choice when you need to add new fields in the table struct. But you should not remove fields in the middle of field-list in which case it could not retrieve table.

Actually this one is newer:)