#### Category: tech

MongoDB read preferences give you control over read behavior when using replicasets. Writes in every environment go to primary, but reads can be configured to read from secondary or primary based on various criteria. In most versions of mongo, the read preference defaults to primary in the client but you should check your version for […]

## Why the words in “CAP theorem” are so confusing!

Consistency The word “consistency” is extremely overloaded in the realm of distributed systems. In CAP, consistency means (informally) that every read reflects the most recent write. This is also known as single-copy consistency or strict / strong consistency. Data is replicated to multiple nodes in these systems, but any reads to this storage system should […]

## SICP Exercise 1.11 – Recursive & Iterative

Prompt A function f is defined by the rule that f(n) = n if n < 3 and f(n)=f(n-1) + 2*f(n-2) + 3*f(n-3) if n >= 3. Write a procedure that computes f by means of a recursive process. Write a procedure that computes f by means of an iterative process. Hint: for iteration, think about how you might code this using a for-loop in a language like […]

## SICP Exercise 1.9 – Process Illustration

Prompt Each of the following two procedures defines a method for adding two positive integers in terms of the procedures inc, which increments its argument by 1, and dec, which decrements its argument by 1 Using the substitution model, illustrate the process generated by each procedure in evaluating (plus-v1 4 5) and (plus-v2 4 5). Are these processes iterative or […]

## SICP Exercise 1.6 – Cond as If?

Prompt Alyssa P. Hacker doesn’t see why if needs to be provided as a special form. “Why can’t I just define it as an ordinary procedure in terms of cond” she asks. Alyssa’s friend Eva Lu Ator claims this can indeed be done, and she defines a new version of if: Eva demonstrates the program for Alyssa: Delighted, Alyssa […]

## Difference between Docker ENTRYPOINT and CMD

ENTRYPOINT and CMD are two docker commands that sound interchangeable, but there are important differences that I’ll cover in this post. I suspect CMD is probably the more familiar instruction, so I’ll go over what that does and how it differs from ENTRYPOINT. Here’s the purpose of CMD, taken straight from the docker manual: The […]

## Distributed Caching Pattern: Write Through

Write-through caching is a caching mechanism where data modifications are written to both the source database and the cache. In this approach, every write operation triggers a write to a RAM cache (such as redis) and the source database (such as postgres). This is commonly used to complement look-aside caching pattern so that when subsequent […]

## Questions to ask before caching with a remote cache

Distributed caching with a system like memcached or redis comes at a cost! If you haven’t added a remote cache yet, here’s some costs to consider: Even if you already operate or use a remote cache, caching any bit of data still adds complexity to your application – and not the essential kind. It’s easy […]

## Forward Proxy vs Reverse Proxy

Two main types of proxies are forward proxies and reverse proxies. Since they’re both proxies, it’s not immediately obvious from their names how they’re different! All proxies act as a middle man in a network topology between two parties: the client (or thing requesting a resource) and a server (the thing providing the resource). The […]

## Distributed Caching Pattern: Cache Aside

A distributed cache is a remote caching system that an application uses via a network to reduce read latency. There are lots of ways an application can interact with this cache and there tends to be common access patterns or strategies – one of the most popular access pattern is called “Cache Aside” (sometimes also […]