# MongoDB Diagnostics and Debugging (Version 3.4) <br />Ch 4: Connectivity ###### tags: `MongoDB University M312` ## What Connectivity Issues Look Like in an Application * Time Outs * Connection Error * Incorrect URI * Non-static IP Addresses * Firewall * Out of conrtrol * DDoS attacks * ISP Performing Poorly ## Timeout ### Timeout Errors * Server Selection TimeoutError * WTimeError ( 僅表示 writeConcern 沒有在時間內完成，不代表 query 沒有完成 ) ```javascript= db.foo.insertOne( {hello: 'world'}, { writeConcern: { w: 3, wtimeout: 2} } ) ``` * Execution Timeout ```javascript= db.collection.find({})._addSpecial("$maxTimeMS", 100) ``` * Network Timeout ## Closing and Dropping Connections * Greedy Connection Applications * Implication of Connection in Memory allocation * How to detect poorly configured hosts * Election of new Primary ### Tools * Mongostat * db.serverStatus() * free -h (OS) * mongod log file :::info MongoDB allows incoming connections up to ***65536***. Since our process require RAM, too many connections is not the normal behavior for a well written application. ::: * **net.maxIncomingConnections** configuration file parameter affect the number of available incoming connections * Cuz one connection is regard as a file to mongod, we need to set higher limit for more connections. ``` bash= # allow open file to 2048 ulimit -n 2048 ``` ```javascript= // shows the available connections of mongod db.serverStatus().connections ``` ### Election of Primary ## Write Concern and Timeouts [Doc](https://docs.mongodb.com/manual/reference/write-concern/#write-concern-specification) :::warning Replica set with 'w:majority' may have some bad situation when there are arbiters or delay servers in the set. ::: ### Hostnames and Cluster Configuration edit host file ```bash= # OS like /etc/host # windows C:\WINDOWS\system32\drivers\etc\hosts ``` ### Networking Best Practices * All Replica Set nodes should be addressable from client hosts and replica set hosts * All mongos nodes should be addressable * Use DNS names instead of IP address to avoid IP address renew resolution conflicts * Make use of ping command to check if node can reach each other * Use telnet to make sure that the ports are available ## Sharding Issues split chunk manually ```javascript= sh.splitAt('m312.example', { last_name: 'E', first_name: Minkey }) ``` Increase chunk size ```javascript= use config db.setting.save( { _id: 'chunksize', value: 150 } ) ``` # MongoDB Diagnostics and Debugging (Version 3.4) <br />Ch 5: Schema Issues ### Schema Investigation 1. turn on the profiler ```javascript= db.setProfilingLevel(2) ``` 2. turn off the profiler and see how many queries were made ```javascript= db.setProfilingLevel(0) db.system.profile.count() ``` 3. Get deeper information ```javascript= db.system.profile.find({}, { planSummary: 1, op:1, ns: 1, query: 1 }).pretty() ```