Detailed Component Design

Common path:

client (using previously described CDN, frontend) sends request to our service. The SSL termination and routing to one of multiple stateless Api-Gateways instances.
Api-gateway performs authentication, authorization and rate-limiting. The rate-limiting data is hold on shared Redis instance (or Cluster for better HA)
- Authentication uses JWT tokens as auth-holder, so it do not requires Database
- The manually expired (in case of logout) JWTs are hold in Redis with TTL calculated to expire token expiration date
The Api-gateway routes request to one of selected service.
- Every service has multiple instances, so to route the request to services fairly, we route to internal proxy (e.g. Haproxy) that balances the load and performs service-discovery (maintain alive services list, performs healthchecks)

Tweet Creation path:

Api-gateway routes the request to one of Tweet Creation service instances.
- if Tweet contains image, the new ID (UUID) is saved to Database
- The image processing is scheduled (async)
- The Tweet is stored in Database and retrieves also the UUID assigned to this tweet
- The Tweet processing is schedule (async)
The response with stored values is returned to current user. However, for now, only user can view newly created Tweet
Image processing:
- There are multiple workers that can handle and can be scaled up/down basing on pending messages metric
- A worker performs resize its and compress image in multiple versions (to adjust to various screens)
- After the worker does it's job it updates image table in database
Tweet processing:
- Tweet needs preprocessed image, so it awaits for image processing to finish using Enrich Pattern (may reach Database Replica for it)
- To make tweet inserted only once, we have unique key on (follower_id, tweet_id)
- Now the processor performs batch insert to all followers timelines (using INSERT INTO select followers SQL query)
- Query for subscribed followers. Information about tweet is also propagated to Redis, so we need to add it also into followers timeline sorted_set.

Timeline retrieval:

Api-gateway routes the request to one of Timeline Services.
If timeline has the opaque_id, we need to access Redis to deserialize it to offset_id.
The timeline service reads from Redis another available entries
if there are no more entries in Redis, we query Database for elder entries.

The TTL for Tweets on timeline is 7 days from creation.

Edge cases:

for very famous creators (e.g. celebrities) the other path is required - pull-based.