[SalesForce] how to retreive missed replay ids in platform events

This may clarify to an extent. Both Platform Events and Streaming API are based on Bayeux protocol and CometD. This is something what the documentation link on Streaming API states on the replayId:

Each broadcasted event is assigned a numeric ID. IDs are incremented and not guaranteed to be contiguous for consecutive events. Each ID is guaranteed to be higher than the ID of the previous event. For example, the event following the event with ID 999 can have an ID of 1,025. The ID is unique for the org and the channel. The IDs of deleted events aren’t reused.

And this is what the documentation on Platform Event says:

The process of replaying platform events is the same as for other Streaming API events

However not really sure, what's the limit of the ids generated, as say it reaches 9999, does it reset to 1 or goes to 10000. But assuming it's a system field, it must have been accounted for a larger limit. But still, I would think these ids should not be used for any uniqueness outside of the event scope.

UPDATE

I have also discussed this topic with some more detail on on my blog here.

Based on your requirements where:

External System will push data in Salesforce.

My recommendation is to construct REST APIs for this purpose which the External Systems can utilize to send data to Salesforce. Platform Events are more targeted towards a Pub/Sub model, where you may want to "trigger" a notification from/to Salesforce to all subscribers and based on that notification the subscriber then takes necessary action. Excerpt from the documentation:

Platform events simplify the process of communicating changes and responding to them without writing complex logic. Publishers and subscribers communicate with each other through events. One or more subscribers can listen to the same event and carry out actions.

If at all you want the External System to "act upon" only when an event has occurred in Salesforce, you can then have Platform Events in this case, where the External System (the subscriber) is subscribed to the events in Salesforce and then sends data to Salesforce. Even in this case, you still need an API exposed for the External System to send data in Salesforce.

One of the most important factors to consider here is what is your integration approach. If you want a more "traditional integration", say point to point (not going into M/W details here), and that the External System always knows at what time they need to send data to Salesforce, go for REST API.

If you want your External System to notify you or if you want the External System to be notified for it to initiate the operation, that is when you should consider the event route (pub/sub).

Hope this helps.