summarize monitoring system

monitorSystem.md

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+# Monitor system
+
+<!-- MarkdownTOC -->
+
+- [Rate limiter](#rate-limiter)
+	- [Scenario](#scenario)
+	- [Storage](#storage)
+	- [Initial solution](#initial-solution)
+	- [Final solution](#final-solution)
+- [Data dog](#data-dog)
+	- [Scenario](#scenario-1)
+	- [Storage](#storage-1)
+
+<!-- /MarkdownTOC -->
+
+## Rate limiter
+### Scenario
+* Limit according to IP / UserId / Email
+* If passing certain times / certain period, return 4XX error codes.
+	- 2/s, 5/m, 10/h, 100d
+	- Do not need too fine granularity
+
+### Storage
+* Need to log who did what at when
+	- Event + feature + timestamp as memcached key
+		+ event = url_shorten
+		+ feature = 192.168.0.1
+		+ timestamp
+* Memcached
+	- Fast
+	- Do not need persistence
+
+### Initial solution
+* Algorithm
+	- memcached.increment(key, ttl=60s)
+	- Increment corresponding bucket, set invalid after 60s
+* Check whether over the limits in the last minute
+
+```
+// Check the visiting sum in the last minute
+for t in 0~59 do
+	key = event + feature + (current_timestamp - t)
+	sum += memcached.get(key, default=0)
+```
+
+### Final solution
+* Multi-level bucket
+	- Use 1 minute as a unit 
+	- Use 1 day as a unit
+
+## Data dog
+### Scenario
+* A user visiting a link is recorded as once visiting.
+* Know the total number of visits.
+* Can check the latest X day/month/years
+* Typically a user always asks from a certain ts to current
+
+### Storage
+* NoSQL system
+	- Need persistent storage
+	- key is tinyUrl short_key, value is the visiting statistics
+* Reduce 2K write QPS by aggregation: 
+	- Need to aggregate the latest 15s visiting records together and then save in memory.
+* Retention: Multi-level bucket
+	- Use 1-minute as a unit
+	- Use 5-minute as a unit
+	- use 1-hour as a unit