Columnar Databases Performance Characteristics Discussions

A discussion detailing performance characteristics of columnar databases, and what situations they are better and what situations they are not.

https://news.ycombinator.com/item?id=3524437

Postgres Query for Current Running Queries

In short, query the pg_stat_activity table

Reference: http://chrismiles.info/systemsadmin/databases/articles/viewing-current-postgresql-queries/

JSch Error "com.jcraft.jsch.JSchException: reject HostKey" Even After Host Key Has Been Added

Usually when this error happens, all you need to do is to add the host key in a normal SSH session. However, this can also happen when the host key is in a format that JSch does not recognise or support e.g. I had a problem with a "ecdsa-sha2-nistp256" encoded host key.

To fix this I had to use the "ssh-keyscan -t rsa <hostname>" command (on the server) to obtain the host key in rsa format and then I manually added that to the "known_hosts" file.

Reference: http://anahorny.blogspot.sg/2013/05/solution-for-comjcraftjschjschexception.html

RHEL / CentOS Tomcat Missing Logs

Temporary workaround is to use "journalctl". Still trying to figure out the long-term solution.

References:

• http://unix.stackexchange.com/questions/224380/tomcat-7-not-producing-log-output-to-catalina-out-on-rhel7
• http://serverfault.com/questions/127640/where-are-my-tomcat-logs

Amazon Making EBS Volume Available for Use

Instructions here: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-using-volumes.html

Linux List Attached Drives

After adding a drive, it will not be visible to the OS until it is mounted and attached. To view it, use the "lsblk" command.

To check the filesystem status of the drive, use the "file -s /dev/<device>" command. If the response is "data". It means the drive is unformatted.

SCP Stalling

In some situations, SCP can stall after just around 5-10 seconds, and it happens consistently. This can sometimes be solved by limiting the bandwidth SCP asks for.

Reference: http://askubuntu.com/questions/82984/why-does-scp-get-stalled-how-do-i-resolve-it

Postgres: "invalid command \N" During Restore

This is sometimes a red herring, where the actual error is higher up on the restore script.

Reference: http://stackoverflow.com/questions/20427689/psql-invalid-command-n-while-restore-sql

iOS Test Flight

To enable Test Flight:

1. Upload a build to App Store (via XCode)
2. Login to iTunes Connect, go to the "Pre-Release" section
3. Enable Test Flight for the build
4. Add users

References:

• http://www.raywenderlich.com/101790/ios-beta-testing-with-testflight-tutorial
• https://itunespartner.apple.com/en/apps/videos#testflight-beta-testing-tutorial

Installing AWS CLI Tools

Not all of AWS features are available in the web console. AWS CLI tools is necessary for calling some of the APIs.

Instructions here: https://github.com/aws/aws-cli

AWS S3 Static Website HTTPS

While it is possible to host static websites on AWS S3, additional steps are necessary to enable SSL with your own custom domain.

In order to do that, you will need to create a CloudFront distribution (CloudFront is a CDN service).

The toughest part is uploading the SSL cert, which has to be done by command line API. Guide here: http://knightlab.northwestern.edu/2015/05/21/implementing-ssl-on-amazon-s3-static-websites/

More details on uploading SSL cert: http://docs.aws.amazon.com/IAM/latest/UserGuide/ManagingServerCerts.html

Commands for uploading cert: http://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_server-certs_manage.html#UploadSignedCert

Note: remember to specify path

References:

• https://aws.amazon.com/blogs/aws/server-name-indication-sni-and-http-redirection-for-amazon-cloudfront/
• http://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/SecureConnections.html#CNAMEsAndHTTPS

C++: Iterating Through Vector of Pointers

(*iter)->someMethod()

Reference: http://stackoverflow.com/questions/23318875/iterating-through-a-vector-of-pointers

Git: Amend Most Recent Commit Message

git commit --amend

Reference: http://stackoverflow.com/questions/179123/edit-an-incorrect-commit-message-in-git

MySQL: Show Index For Table

"SHOW INDEX FROM table"

Reference: https://dev.mysql.com/doc/refman/5.0/en/show-index.html

Bash IO Redirection

">" for normal output. "2>" for error output.

Reference: http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO-3.html

Mac App Store Connection Failed

When trying to connect to the Mac App Store, there is the error message "Cannot connect to the app store. An internet connection is required."

There are many documented solutions for this, all of which will probably work for one person or another. However, the only method that worked for me was resetting my keychain: https://support.apple.com/en-sg/HT203192

Other solutions

• https://support.apple.com/en-us/HT203811
• https://josephscott.org/archives/2013/12/mac-os-x-app-store-not-loading-try-resetting-nvram/

C++: Find string within a string

There is a string::find function that is pretty straightforward to use. The key thing to note here is that when it's not found the return value is "string::npos".

Reference: http://www.cplusplus.com/reference/string/string/find/

C++: Constructing a std::string from std::vector

std::vector<char> is actually a vector of the characters that make up the string.

There is a constructor for std::string that can be used for this i.e. std::string s(v.begin(),v.end());

Reference: http://stackoverflow.com/questions/5115166/how-to-construct-a-stdstring-from-a-stdvectorchar

Coco2D-X: long Values on iOS

"long" isn't guaranteed to be 64 bits on iOS. To be sure that there won't be overflow, use int64_t.

Reference http://stackoverflow.com/questions/13604137/definition-of-int64-t

C++ std::string String Concatenation

Reference here: http://stackoverflow.com/questions/662918/how-do-i-concatenate-multiple-c-strings-on-one-line