Wednesday, July 28, 2010

Cheap difference between switch and hub



Some users have recently seen a message that the article to buy according to our networking products, businesses that have switches and hubs, switches more expensive, cheaper hubs, then the two products because they do not know what difference, it is feared by business fraud, they can not make a decision.

For this problem, we will for everyone from the technical and practical aspects on analysis of the low-end switches (in the high-end switches for large-scale professional networking) and the hub of the differences:

Start with the technical analysis:

We now use the methods of work adopted Ethernet is CSMA / CD (Carrier Sense Multiple Access / Collision Detection), the sender, it sends a data message for each, the first monitor on the network, when it detects line just free, they immediately send data, or to continue testing until the line is idle and then send. For the receiver, the received signal first to confirm, if it is issued to own the reception, otherwise ignore. Thus, the data transmission network exists to share and exchange these two concepts - shared management of the network is a non-working state to ease the disorder, each client will be possible to seize the communication channel, so a few clients the time to seize the channel with the formation of network congestion on the situation, when the data and the number of users exceeds a certain limit, they will cause the network performance of a deep recession. The switching network is to avoid the lack of shared networks, the role of the exchange of technology transfer is based on the packet destination address, independently of each packet sent to destination port from the port to avoid collision with other ports, increase the actual throughput of the network.

Shared Ethernet, the main problems is that all users share the bandwidth, the actual available bandwidth for each user with the network reduced the number of users increases. This is because when information is busy, the number of users may be linked into the "race" a channel and a channel at a time occupied only allow one user, so a lot of regular monitoring of wait state is a result, the signal transmission produce jitter, stagnation or distortion, seriously affected the performance of the network. Switched Ethernet, the switch supplies the user-specific information for each channel, unless both the source port information while trying to send to the same destination port, or each individual source port and destination port for communication between not occur simultaneously conflict.

Therefore, the hub is a shared work with the state of the equipment, since it is relatively easy to implement, so the prices are cheaper hubs, of course, deficiencies in speed will be difficult to avoid; while the switch is using switched technology, equipment, technical content higher than the hub, of course, also more expensive the price, but the switch can overcome the shortcomings of network congestion.

To sum up so to speak:

All ports on the hub of a shared channel contention bandwidth, so as to increase the number of network nodes, data traffic increases, the available bandwidth per node will be reduced. Hub in the form of transfer data by radio to all its ports to transmit data.

All ports on the switch are exclusive channel bandwidth to each port to ensure fast and efficient data transfer. Switch is to provide users with exclusive, point to point connection, the packets are only sent to the destination port, and will not send to all ports.

Hub is a shared device, itself can not identify the destination address, of that same period of a host to a web host to send data O'clock b Packets Yi hub Wei structure of the network in Guangbofangshi Chuan Shu, You each terminal by Taiwan Verify header information to determine whether the address to receive the same time the network can only transmit a data frame of the communication. This means shared bandwidth.

Switch based on mac address recognition, to complete the package forwarding data-enabled devices. Switches can learn mac address on the internal address table, the data frame through the origin and the recipient of the switching path between the interim, the data from the source address to destination address.

After reading the above, we should understand why some businesses will directly tell you "faster switches, hubs slow" (the argument is easy to understand it).

Let us go from the application of:

As the hub can not overcome a large amount of data and multi-user when the plug issue, so it can not connect too many clients at the same time we typically see in the market, many dozens of pieces of the hub are five of the ( previous 16 Internet cafe hub is also very popular, but now have been replaced by a switch), and only 10M; the switch 5 or 8 have (24 in all, but home prices are more expensive and really nothing necessary ), 10/100M speed are basically adaptive.

So after you see the article inside another switch or hub that can be used, then the best choice switch it, although a little expensive, but it is really easy to use than many of the hub.







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Wednesday, July 21, 2010

JPA entities of state and API



Entity object has the following four states, these states occur by calling the EntityManager interface method of migration:

New state: the newly created entity object, yet have persistent primary key, and a persistence context does not associate. Controlled state: already have a persistent primary key and and the persistence context to establish a link; free State: have a persistent primary key, but not yet, and persistence context to establish contacts; delete state: has a persistent primary key has been, and persistence context to establish contacts However, arrangements have been deleted from the database.

The API EntityManager

Here are some of the main interface EntityManager methods:

void persist (Object entity)

By calling the EntityManager's persist () method, the new entity instance will be converted to control. This means that when the persist () method where the transaction commits, the entity data will be saved to the database. If the entity has been persistent, then the call to persist () operation does not occur anything. If an entity has been removed calls persist () operation to remove into another state entity controlled state. If the implementation of free-like entities persist () operation will throw IllegalArgumentException.

In an entity called persist () operation will be broadcast to the associated entities and entities on the implementation of the corresponding cascade persistence operations;

void remove (Object entity)

By calling remove () method to remove a controlled entity. If the entity declared as a cascading delete (cascade = REMOVE or cascade = ALL), was associated entities will be deleted. In a new state of the entity called remove () operation will be ignored. If the free entity called remove () operation throws IllegalArgumentException, related to the transaction will be rolled back. If the entity has been removed to perform remove () operation will be ignored;

void flush ()

The controlled state of the entity data synchronization to the database;

T merge (T entity)

A free state of the entity persistence to the database, and converted into a controlled state of the entity;

T find (Class entityClass, Object primaryKey)

To the primary key query entity objects, entityClass is the entity class, primaryKey is the primary key value, such as the following code to query entities Topic:

Topic t = em.find (Topic.class, 1);

Query createQuery (String qlString)

According to JPA's query to create a query object Query, such as the following code:



Query q = em.createQuery ("" SELECT t FROM Topic tWHERE t.topicTitle LIKE: topicTitle ")"); Query createNativeQuery (String sqlString) using a local database SQL statements to create a Query object, Query by getResultList () Method of implementation of the query, returns a List result set, each row of data corresponds to a Vector.
Query

JPA represents a query interface, using javax.persistence.Query instance, Query instance from the EntityManager query by specifying the building. This interface has many methods to perform data query interface:

Object getSingleResult (): implementation of the SELECT query and returns a result;

List getResultList (): implementation of the SELECT query and return multiple results;

Query setParameter (int position, Object value): binding by position number parameter in the parameter query, if the query using the command parameter, you can use Query setParameter (String name, Object value) method of binding named parameters;

Query setMaxResults (int maxResult): Set the maximum number of results returned;

int executeUpdate (): If the query is to add, delete, or change the statement, by the method of implementation of the update operation.







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Monday, July 5, 2010

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