How do I make a required thrift field optional?

Question

What is the best process for making a required field optional in thrift. For example, I have a struct...

struct Message {
    1: required double userID;
    2: required string content;
    ...
} 

... but I want to make content optional.

EDIT: To clarify, I already have consumers that use this struct, so I would need to update it without breaking those consumers. A staged upgrade is fine (ie - add a new optional field, update the downstream clients, then remove--or stop using--the old required field).

Answer 1

It is possible to do this if you control all the readers and writers of the struct in the following steps :

1. Change the field in the thrift schema from required to optional. Sync this new schema in all the systems that refer to or use this struct. Do not stop writing this field. This is a safe change as the field will still be always present.
2. Only after the schema change has been deployed in all the systems using this struct, you can stop writing this field in some cases. This is now safe to do as this field is marked as optional in every system, there are no "old readers" as referred to in the other answer.