check the following tables in your DB:
catalog_product_link_attribute and catalog_product_link_type
If those are empty, you need to run the following SQL queries:
(1, 1, ‘position’, ‘int’),
(2, 4, ‘position’, ‘int’),
(3, 5, ‘position’, ‘int’),
(4, 3, ‘position’, ‘int’),
(5, 3, ‘qty’, ‘decimal’);
A cluster is the group of computing machines that can individually run the software. Clusters are typically utilized to achieve high availability for server software.
Clustering is used in many types of servers for high availability.
App Server Cluster – An app server cluster is the group of machines that can run an application server that can be reliably utilized with a minimum of downtime.
Database Server Cluster – A database server cluster is the group of machines that can run a database server that can be reliably utilized with a minimum of downtime.
Scalability is the ability of a system, network, or process to handle a growing amount of load by adding more resources. The adding of resource can be done in two ways
Scaling Up – This involves adding more resources to the existing nodes. For example, adding more RAM, Storage or processing power.
Scaling Out – This involves adding more nodes to support more users.
Any of the approaches can be used for scaling up/out an application, however,r the cost of adding resources (per user) may change as the volume increases. If we add resources to the system It should increase the ability of the application to take more load in a proportional manner of added resources.
An ideal application should be able to serve high level of load in fewer resources. However, in a practical, linearly scalable system may be the best option achievable.
Poorly designed applications may have a really high cost on scaling up/out since it will require more resources/user as the load increases.
Having better service capacity with high availability and low latency is mission critical for almost all businesses.
Availability means the ability of the application used to access the system, If a user cannot access the application, it is assumed unavailable. High Availability means the application will be available, without interruption.
Achieving high availability for an application is not always an easy task. Using redundant server nodes with clustering is a common way to achieve a higher level of availability in web applications.
Availability is commonly expressed as a percentage of uptime in a given year.
Sharding is an architectural approach that distributes a single logical database system into a cluster of machines.
Sharding is Horizontal partitioning design scheme. In this database design rows of a database table are stored separately, instead of splitting into columns (like in normalization and vertical partitioning). Each partition is called a shard, which can be independently located on a separate database server or physical location.
Sharding makes a database system highly scalable. The total number of rows in each table in each database is reduced since the tables are divided and distributed into multiple servers. This reduces the index size, which generally means improved search performance.
The most common approach for creating shards is by the use of consistent hashing of a unique id in the application (e.g. user id).
The downsides of sharding are,
- It requires the application to be aware of the data location.
- Any addition or deletion of nodes from the system will require some rebalance to be done in the system.
- If you require a lot of cross-node join queries then your performance will be really bad. Therefore, knowing how the data will be used for querying becomes really important.
- A wrong sharding logic may result in worse performance. Therefore make sure you shard based on the application need.
Session replication is used in application server clusters to achieve session failover.
A user session is replicated to other machines of a cluster, every time the session data changes.
If a machine fails, the load balancer can simply send incoming requests to another server in the cluster.
The user can be sent to any server in the cluster since all machines in a cluster have a copy of the session.
Session replication may allow your application to have session failover but it may require you to have extra cost in terms of memory and network bandwidth.
Failover means switching to another machine when one of the machines fails.
Failover is an important technique in achieving high availability. Typically a load balancer is configured to fail over to another machine when the main machine fails.
To achieve the least downtime, most load balancers support a feature of heartbeat check. This ensures that the target machine is responding. As soon as a heartbeat signal fails, load balancer stops sending the request to that machine and redirects to other machines or cluster.
IP address affinity is another popular way to do load balancing. In this approach, the client IP address is associated with a server node. All requests from a client IP address are served by one server node.
This approach can be really easy to implement since the IP address is always available in an HTTP request header and no additional settings need to be performed.
This type of load balancing can be useful if your clients are likely to have disabled cookies.
However, there is a downside to this approach. If any of your users are behind a NATed IP address then all of them will end up using the same server node. This may cause an uneven load on your server nodes.
NATed IP address is really common, in fact, anytime you are browsing from an office network its likely that you and all your coworkers are using same NATed IP address
In a load balanced server application where user information is stored in the session, it will be required to keep the session data available to all machines. This can be avoided by always serving a particular user session request from one machine.
The machine is associated with a session as soon as the session is created. All the requests in a particular session are always redirected to the associated machine. This ensures the user data is only at one machine and load is also shared.
In the Java world, this is typically done by using a jsessionid cookie. The cookie is sent to the client for the first request and every subsequent request by a client must be containing that same cookie to identify the session.
There are few issues that you may face with this approach
- The client browser may not support cookies, and your load balancer will not be able to identify if a request belongs to a session. This may cause strange behavior for users who use no cookie based browsers.
- In case one of the machines fails or goes down, the user information (served by that machine) will be lost and there will be no way to recover user session.
The PHP parsing engine needs a way to differentiate PHP code from other elements in the page. The mechanism for doing so is known as ‘escaping to PHP’. Escaping a string means to reduce ambiguity in quotes used in that string.