Category Archives: {Code}

The Problem

There are times when you want to fetch data from your server that is occasionally changed. For example, getting the number of unread messages in an application. There are a few possible solutions for keeping the data up-to-date with the backend. 1) You can poll the server every X minutes, or 2) you can use “cache-and-network” to utilize the cache first, but also check the server for updated data.

Polling

If you choose to poll for new messages, you run the risk of your application constantly calling the server for data even though the user has not recently interacted with the page. Think, checking another tab or walking away from the computer for a while. While not detrimental, this results in a lot of wasted calls and network requests.

Cache and Network

This method ensures you always have the most up-to-date information from the server, but also results in a lot of needless calls to the server. The client is constantly saying, “Ok, I’ll use the cache, but let me still call the server to check for updated data.” This query/hook may run multiple times on a single user interaction and is still to aggressive for data that only updates every few minutes.

The Solution

What we really want is the best of both worlds. We want to utilize the cache as much as possible, but also set an expiration for when to grab fresh data from the server. We can use a simple technique of utilizing Map and dates to accomplish this goal.

We’ll simply make a unique “key” for our query and record the last time we fetched that key. When we call useQuery, we pass it our “key” and the expected expiration time. If the “key” is still valid, we return to useQuery the fetchPolicy of “cache-first.” This means, use the cache if it exists and otherwise fetch from the server. If the “key” is invalid, we call useQuery with “network-only” which calls the server to refresh the data. And at the same time, we update the last fetched time of the key.

Let’s take a look at an example of the getFetchPolicyForKey() function and its implementation.

useUnreadMessages.ts

And here’s the implementation of the cache check and when to return each fetch policy.

getFetchPolicyForKey.ts

Let’s say you’re sending a button from a parent to a child using render props.  How do you properly render and place the returned button?

As it turns out, its quite easy.

Let’s say that this is your component which uses render props to return both the toggle state and the toggle button.

To use the above provider, you may be tempted (because of patterns) to do this.

However, because the parent has already rendered the button, the child needs only to place the rendered component as if it were any other variable.

That’s it.

 

I was recently adding Flow typing to a HoC and ran into a specific use case.  The HoC injected its own function into the child component while also using of the of child props in the injected method.  So here's how to properly flow type it so that consumer of the HoC will enforce the typing of the child.

First, add the type for the prop you will be injecting into the child Second, add the type for the prop you need to use from the child Last, update the HoC wrapper as follows  

Here's what each of those lines is doing:

This is ensuring that for the calling of this HoC, we enforce the typing of the child and add the requirement for our own props. This simply passes along the typing of the child component. This declares that we will return the prop types of the child, minus the injected props we will provide. Putting it all together, here is our final flowtyped HoC.

For a weekend project I built a backlight array for my TV which gives off ambient lighting according to what colors are being shown across the edges of the screen. Powered by a microduino and Abalight software for Mac.

According to the experts, backlights offset the brightness of the screen by lighting up the entire wall, while also giving the impression that the movie is bigger than just the screen.  I just say “woot.”

bin-bash

I recently had an issue with a server hosted at OVH.com.   One Saturday, it completely failed to boot and there was no SSH access.  After a little pleading with OVH to fix the server(since I had no access) , I was flatly told “No” several times. This was a dedicated server and as such, I was responsible for all aspects of its software.  “But your physical disks are failing to mount!” No help.  Grrr, ok I can solve this.

Here’s the only problem, I don’t even know what the error is on boot! I can’t see any of the boot screen, so I don’t know where its failing.

OVH has a convenient feature know as Rescue Mode which allows you to boot to an alternative  OS, so you can mount and correct any issues on the primary drive.  Utilizing this feature I got access to the disks and RAID array.  Everything seemed fine, but I ran through all the checks to be sure.

  • Hard Disks – No errors
  • Raid Array – Needed to be resynced, but does not fix boot
  • FileSystem – OK
  • Boot Logs – No help
  • Partitions – Disks are out of order, but does not fix boot

At this point, I’m out of ideas, so I call OVH one more time and ask them to look at the boot screen and tell me where its stuck.  They agree and tell me it boots to GRUB> prompt and stops.  Ok, this is good information.

I log back into rescue and scour the internet for a way to fix this.  The answer is found in an obscure ubuntu forum, which perfectly describes a way to reset the grub loader on each disk in the array, when utilizing a rescue mode.

$ sudo fdisk -l (From this you need to find the device name of your physical drive that won't boot, something like “/dev/sdxy″ - where x is the drive and y is the root partition. Since I was using a software RAID, root (/) was on md1)
$ sudo mount /dev/sdxy /mnt (Mount the root partition)
$ sudo mount --bind /dev /mnt/dev
$ sudo mount --bind /proc /mnt/proc
$ sudo mount --bind /sys /mnt/sys
$ sudo chroot /mnt  (This will change the root of executables to your your drive that won't boot)
$ grub-mkconfig -o /boot/grub/grub.cfg (insure that there are NO error messages)
$ grub-install /dev/sdx (NOTE that this is the drive and not the partition. try grub-install --recheck /dev/sdxy if it fails)
Ctrl+D (to exit out of chroot)
$ sudo umount /mnt/dev
$ sudo umount /mnt/proc
$ sudo umount /mnt/sys
$ sudo umount /mnt

Reboot!

Hopefully this will save someone some agony in the future and give you a few hours of your life back.

nginx

I recently need to set up caching for a very slow API service I was working with.  However the app I was working with forced a query param of “timestamp” on all requests.  This effectively killed the cache because all api requests were flowing through a single endpoint, and only the query params differentiated the requests.  So here’s how to set up caching and strip out query params with nginx.