A service mesh is an infrastructure layer built into an app which documents how services interact making it easier to optimize communication and avoid downtime.
A service mesh, like the open source project Istio, is a way to control how different parts of an application share data with one another. Unlike other systems for managing this communication, a service mesh is a dedicated infrastructure layer built right into an app. This visible infrastructure layer can document how well (or not) different parts of an app interact, so it becomes easier to optimize communication and avoid downtime as an app grows.
Each part of an app, called a "service," relies on other services to give users what they want. If a user of an online retail app wants to buy something, they need to know if the item is in stock. So, the service that communicates with the company's inventory database needs to communicate with the product webpage, which itself needs to communicate with the user’s online shopping cart. To add business value, this retailer might eventually build a service that gives users in-app product recommendations. This new service will communicate with a database of product tags to make recommendations, but it also needs to communicate with the same inventory database that the product page needed—it’s a lot of reusable, moving parts.
Modern applications are often broken down in this way, as a network of services each performing a specific business function. In order to execute its function, one service might need to request data from several other services. But what if some services get overloaded with requests, like the retailer’s inventory database? This is where a service mesh comes in—it routes requests from one service to the next, optimizing how all the moving parts work together.